UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS. 

COLLEGE  OF  AGRICULTURE. 

AGRICULTURAL  EXPERIMENT  STATION. 

BERKELEY,  CALIFORNIA. 


GUM  DISEASE  OF  CITRUS  TREES 
IN  CALIFORNIA. 

By  RALPH  E.  SMITH  and  0.  BUTLER. 


BULLETIN    No.    200 

(Berkeley,  Cal.,  August,  1908.) 


SACRAMENTO 

w.  w.  shannon,      :     :     :     :      superintendent  state  printing 

1908 


BENJAMIN  IDE  WHEELER,   Ph.D.,  LL.D.,  President  of  the   University. 


EXPERIMENT  STATION  STAFF. 

E.  J.   WICKSON,  M.A.,  Director  and  Horticulturist. 

E.  W.  HILGARD,  Ph.D.,  LL.D.,  Chemist. 

W.  A.  SETCHELL,  Ph.D.,  Botanist. 

ELWOOD  MEAD,  M.S.,  C.E.,  Irrigation  Engineer.      (Absent  on  leave.) 

LEROY  ANDERSON,  Ph.D.,  Dairy  Industry  and  Superintendent  University  Farm. 

M.  E.  JAFFA,  M.S.,  Nutrition  Expert,  in  charge  of  the  Poultry  Station. 

C.  W.  WOODWORTH,  M.S.,  Entomologist. 

R.  H.  LOUGHRIDGE,  Ph.D.,  Soil  Chemist  and  Physicist. 

G.   W.   SHAW,  M.A.,   Ph.D.,  Experimental  Agronomist  and  Agricultural  Technologist, 
in  charge  of  Cereal  Stations 

GEORGE  E.   COLBY,  M.S.,   Chemist,  in  charge  of  Agricultural  Chemical  Laboratory. 
RALPH  E.  SMITH,  B.S.,  Plant  Pathologist  and  Superintendent  of  Southern  California 

Pathological  Laboratory  and  Experiment  Station.      Whittier. 
A.   R.   WARD,   B.S.A.,   D.V.M.,   Veterinarian  and  Bacteriologist. 

E.  W.  MAJOR,  B.Agr.,  Animal  Industry. 

H.  M.  HALL,  M.S.,  Assistant  Botanist. 

H.  J.  QUAYLE,  A.B.,  Assistant  Entomologist.     Whittier. 

W.    T.    CLARKE,    B.S.,    Assistant    Horticulturist    and    Superintendent    of    University 

Extension  in  Agriculture. 
JOHN  S.  BURD,  B.S.,  Chemist,  in  charge  of  Fertilizer  Control. 
C.  M.  HARING,  D.V.M.,  Assistant  Veterinarian  and  Bacteriologist. 
H.  A.  HOPPER,  M.S.,  Assistant  in  Dairy  Husbandry. 
J.  H  NORTON,  M.S.,  Assistant  Chemist  in  charge  Fertilizer) 

Exyerim ents,  [      Citrus  Experiment 

T.  F.  HUNT,  B.S.,  Assistant  Horticulturist,  )  Station,  Riverside. 

E.  B.  BABCOCK,  B.S.,  Assistant  Plant  Pathologist. 

E.  H.  SMITH,  M.S.,  Assistant  Plant  Pathologist. 

F.  L.  YEAW,  B.S.,  Assistant  Plant  Pathologist. 

H.  J.  RAMSEY,  M.S.,  Assistant  Plant  Pathologist,  \        Southern  California  Patholog- 

C.  O.    SMITH,   M.S.,  "  "  "  )  ical  Laboratory,  Whittier. 
R.   E.    MANSELL,   Assistant  in  Horticulture,  in  charge   of   Central   Station    Grounds. 
RALPH  BENTON,  B.S.,  B.L.,  Assistant  in  Entomology   {Apiculture) . 

A.  J.  GAUMNITZ,  M.S.,  Assistant  in  Cereal  Investigations. 

RACHAEL  CORR,  M.A.,  Assistant  in  Cereal  Laboratory. 

HANS  C.  HOLM,  B.S.,  Assistant  in  Zymology. 

P.  L.  McCREARY,  B.S.,  Laboratory  Assistant  in  Fertilizer  Control. 

F.  E.  JOHNSON,  B.  L.,  Assistant  in  Soil  Laboratory. 

M.  E.  STOVER,  B.S.,  Assistant  in  Agricultural  Chemical  Laboratory. 

D.  R.  HOAGLAND,  A.B.,  Assistant  in  Agricultural  Chemical  Laboratory. 
CHARLES  FUCHS,  Curator  Entomological  Museum. 

P.  L.  HIBBARD,  B.S.,  Assistant  Fertilizer  Control  Laboratory. 

M.  E.  SHERWIN,  Field  Assistant  in  Agronomy. 

W.  H.  VOLCK,  Field  Assistant  in  Entomology.     Watsonville. 

E.  L.  MORRIS,  B.S.,  Field  Assistant  in  Entomology.     San  Jose. 
J.  S.  HUNTER,  Field  Assistant  in  Entomology.     San  Mateo. 

D.  L.  BUNNELL,  Clerk  to  the  Director. 


JOHN  TUOHY,  Patron,  )  Tulare  8uhstation>  Tulare 

J.  T.  BEARSS,  Foreman,  \ 

J.  W.  ROPER,  Patron,  )  University  Forestry  Station,  Chico. 

E.  C.  MILLER,  In  charge,  ) 


University  Forestry  Station,  Santa  Monica. 


ROY  JONES,  Patron,  ) 

N.  D.  INGHAM,   Foreman,    \ 
VINCENT     J.      HUNTLEY,     Foreman     of     California    Poultry     Experiment     Station, 
Petaluma. 


The  Station  publications  (Reports  and  Bulletins),  so  long  as  avail- 
able,  will  be  sent  to  any  citizen  of  the  State  on  application. 


GUM  DISEASE  OE  CITRUS  TREES  IN  CALIFORNIA 

By  RALPH  E.    SMITH  and   O.    BUTLER. 


INTRODUCTORY. 

The  most  common  and  characteristic  citrus  tree  diseases  in  California 
are  those  which  may  be  included  under  the  general  term  "Gum  Dis- 
ease." Although  the  troubles  which  we  shall  consider  are  known 
locally  under  a  variety  of  names,  they  all  have  the  one  feature  in  com- 
mon of  an  abnormal  production  of  tree  gum  from  the  affected  parts, 
and  so  may  be  included  in  the  same  category.  As  we  shall  show  at  the 
proper  time,  this  characteristic  of  gumming  is  not  the  only  one  com- 
mon to  these  various  troubles. 

To  a  certain  extent  the  various  forms  of  gum  disease  are  serious,  and 
yet  they  are  by  no  means  disastrous ;  their  nature  is  in  a  sense  unknown, 
and  yet  for  practical  purposes  fairly  well  understood;  their  control 
is  often  difficult  and  by  no  means  generally  accomplished,  yet  almost 
always  successfully  attained  when .  taken  in  time  by  proper  means, 
when  conditions  impossible  of  improvement  are  not  present.  In  short, 
gum  disease  is  one  of  those  conditions  which  require  careful,  continued, 
intelligent  work  along  various  simple  lines  for  successful  handling, 
rather  than  something  of  a  strikingly  specific  nature  with  quick  evi- 
dences of  action  and  effect.    It  is  also  easier  of  prevention  than  of  cure. 

The  present  bulletin  will  consider  the  various  citrus  troubles  which 
may  be  included  in  the  category  of  gum  diseases,  showing  the  features 
which  they  have  in  common  and  discussing  their  nature,  together  with 
methods  of  prevention  and  cure.  Since  the  founding  of  a  laboratory 
of  plant  pathology  in  southern  California  opportunity  has  been  afforded 
for  a  thorough  study  of  the  gum  disease  problem  in  the  field  and  labora- 
tory under  the  most  favorable  conditions.  At  the  same  time  many 
growers  and  other  interested  persons  have  been  experimenting  in  a 
variety  of  ways  in  the  practical  treatment  of  the  disease,  so  that  a 
considerable  fund  of  information  is  available,  based  on  the  experiences 
of  others.    This  is  freely  drawn  upon  in  the  present  bulletin. 

From  our  own  field  work  and  the  observations  of  others,  taken  in 
connection  with  the  laboratory  study  which  we  have  made,  we  feel 
ready  to  speak  with  a  considerable  degree  of  certainty  upon  the  subject. 
Further  studies  are  still  desirable  and  are  being  carried  on  from  the 
standpoint  of  advanced  plant  physiology  and  pathology  as  to  the  exact 
nature  of  the  influences  and  processes  which  bring  about  the  character- 
istic effects  of  gum  disease. 


236 


UNIVERSITY   OP   CALIFORNIA — EXPERIMENT   STATION. 


ANATOMY  AND  PHYSIOLOGY  OF  THE  CITRUS  TREE. 

This  account  of  gum  disease  is  based  on  the  belief  that  no  parasitic 
or  infectious  organism  is  primarily  concerned  in  causing  the  trouble, 
but  that  the  various  forms  of  the  disease  are  the  result  of  physiological 
disturbances  in  the  functions  of  the  tree.     A  clear  understanding;  of 


fti'i'l'Wl'lVi'i'i 


'"J'i'i'V.1    I 


Fig.  1.  Structure  of  citrus  stem.  W  wood,  C  cambium  layer,  B  bark, 
G  region  of  gum  formation.  Arrows  indicate  upward  flow  of  sap 
in  wood  and  downward  path  of  nutritive  material  in  bark. 

the  principles  of  plant  physiology  and  structure  is  therefore  necessary 
to  a  proper  conception  of  the  nature  of  the  disease. 

The  citrus  tree,  like  other  similar  plants,  consists  of  certain  parts, 
of  which  the  most  important  in  the  present  connection  are  the  root, 
stem  and  branches,  and  the  leaves. 


Bulletin  200.  GUM  disease  OF  CITRUS  TREES.  237 

The  Boot. — The  root  is  the  organ  of  absorption  of  water  and  mineral 
food  elements,  and  likewise  acts  mechanically  in  holding  the  plant  in 
the  soil.  It  subdivides  extensively,  resolving  at  its  extremities  into  the 
finest  hairlike  branches  and  outgrowths.  Through  the  root  is  obtained 
water  from  the  soil,  with  nitrates,  phosphates,  potash,  iron,  lime,  etc., 
in  solution,  by  a  process  of  absorption.  All  the  water  used  by  the  tree 
is  obtained  in  this  way  from  the  soil. 

Upward  Flow  of  Sap. — This  watery  solution  or  crude  sap  passes  into 
the  woody  portion  of  the  root  and  thence  travels  up  into  the  trunk, 
the  stream  being  located  mainly  in  the  younger,  outer  part  of  the  wood 
or  sap-wood.  There  is  no  rapid  flowing  along  of  this  water  up  into 
the  tree  from  the  root,  like  the  blood  circulation  of  animals.  The  cur- 
rent is  a  considerable  one,  however,  particularly  in  the  citrus  tree,  and 
the  sap  passes  through  certain  comparatively  large  tubular  pores  or 
vessels  in  the  wood,  which  are  formed  for  this  purpose.  Citrus  trees 
are  particularly  quick  to  respond  to  the  application  of  water  to  the 
roots,  sending  large  amounts  up  through  the  trunk  very  soon  after  the 
application  is  made. 

The  Leaves. — While  the  root  is  engaged  in  absorbing  and  pumping 
up  water  and  mineral  salts,  the  leaves  are  equally  active  in  doing  their 
part  in  the  nutrition  of  the  tree.  During  sunlight  the  leaves  constantly 
absorb  carbon  dioxide  gas  from  the  air,  and,  by  a  remarkable  activity 
which  they  possess,  they  decompose  this  gas  and  unite  its  carbon  with 
the  hydrogen  and  oxygen  derived  from  the  water  furnished  by  the 
roots,  forming  starch.  In  this  way  the  green  leaves  of  plants  accumulate 
large  amounts  of  starch  during  the  daytime  and  are  charged  with  this 
substance  when  evening  comes. 

Digestion. — Starch  being  a  solid  material,  it  is  necessary  that  it  be 
made  soluble  before  it  can  pass  out  of  the  leaves  or  be  further  utilized 
by  the  plant.  This  process  takes  the  place  in  the  same  manner  as  in  ani- 
mals, namely,  by  the  action  of  the  substance  called  diastase,  which 
converts  starch  into  sugar  and  thus  makes  it  soluble  in  water.  Further 
changes  of  a  very  complicated  nature  then  go  on  in  the  nutritive 
materials,  through  which  a  combination  is  effected  of  those  materials 
obtained  respectively  by  the  roots  and  leaves,  forming  various  soluble 
substances  of  a  nutritive  nature. 

During  these  changes  most  of  the  water  taken  up  by  the  roots  is 
carried  up  to  the  leaves,  and  this  process  constantly  goes  on,  even  in 
the  tallest  trees.  A  constant  and  abundant  flow  of  crude  sap  takes 
place,  passing  through  the  outer  layers  of  wood,  as  previously  described, 
dividing  and  subdividing  into  the  various  branches  and  twigs,  and 
finally  reaching  the  leaves.     At  the  same  time  all  parts  of  the  tree 


238  UNIVERSITY   OF   CALIFORNIA EXPERIMENT   STATION. 

absorb  more  or  less  of  the  soil  water,  by  its  movement  in  all  directions, 
sufficient  to  prevent  wilting  and  drying  out. 

Downward  Flow  of  Sap. — After  the  nutritive  material  has  been  prop- 
erly prepared  as  described  above,  it  is  distributed  again  to  all  parts 
of  the  plant,  in  the  form  of  a  solution  in  water.  This  distribution 
takes  place  mainly  by  a  current  passing  through  the  inner  bark,  and 
thus  we  have  in  the  trunk,  to  a  certain  extent,  two  currents  of  sap 
moving  in  opposite  directions,  one  going  upward  through  the  wood, 
carrying  crude  food  material,  and  the  other  coming  downward  through 
the  inner  bark,  furnishing  digested  material  for  the  growth  and  nutri- 
tion of  the  tree.  The  latter  process,  however,  does  not  consist  in  an 
abundant  flow  of  sap  traveling  rapidly  in  one  direction,  but  is  rather 
a  slow  process  of  absorption  and  diffusion  of  the  nourishing  materials. 

Cambium  Layer. — Just  between  the  bark  and  wood  is  situated  the 
most  vital  part  of  the  tree,  the  cambium  layer,  which  is  the  seat  of  growth 
in  thickness.  This  area,  consisting  of  a  narrow  layer  of  tissue  running 
around  the  trunk  and  all  the  branches  just  between  the  wood  and  bark, 
gradually  grows  out  with  the  expansion  of  the  tree,  forming  the  elements 
of  the  wood  on  its  inner  side  and  those  of  the  bark  on  the  outer  side. 
To  the  cambium  goes,  therefore,  much  of  the  digested  food  material, 
and  it  is  used  in  the  formation  of  the  tissues  mentioned,  just  as  the 
blood  in  animal  life  conveys  nutritive  matter  for  utilization  in  the 
growth  and  maintenance  of  the  various  tissues  of  the  body. 

The  account  here  given  is  but  a  brief  one,  and  does  not  consider  all 
the  numerous  and  obscure  processes  which  go  on  in  the  formation,  move- 
ments, and  chemical  changes  of  these  food  materials.  Enough  has  been 
said,  however,  to. show  the  extremely  complicated  and  delicate  nature 
of  the  process,  and  the  fact  that  in  plants,  as  well  as  in  animals,  com- 
plications of  the  nature  of  indigestion  and  other  failures  or  abnormal 
effects  in  the  vital  processes  may  easily  occur.  This  is  particularly  the 
case  in  the  citrus  tree,  which  is  of  a  very  susceptible  nature  with  an 
abundant  flow  of  sap,  and  is  grown  in  California  under  decidedly  arti- 
ficial conditions,  with  the  supply  of  moisture  and  nutriment  largely 
controlled  by  the  grower  rather  than  by  favorable  influences  of  nature. 
Processes  of  oxidation,  respiration,  excretion,  and-  other  complicated 
chemical  and  physiological  changes  also  go  on  in  the  citrus  tree,  and 
these  are  easily  affected,  adversely  or  otherwise,  by  climatic,  soil  and 
cultural  conditions. 

GENERAL   NATURE   OF   GUM   DISEASE. 
Gum  disease  or  (iummosis  of  trees  is  a  term  applied  to  a  condition  in 
which  an  exudation  of  gummy  sip  takes  place  through  the  bark.    Such  a 
flow  of  gum  may  be  accompanied  by  other  symptoms  more  or  less  pro- 
nounced, and  is  almost  always  connected  willi  a  dying  or  unhealthy  con- 


Bulletin  200.  GUM  disease  OF  CITRUS  TREES.  239 

dition  of  the  tree.  In  other  words,  the  gumming  is  a  symptom  of  injury 
or  disease  and  not  a  normal  function.  Such  troubles  are  particularly 
characteristic  of  two  classes  of  trees :  citrus  and  stone  fruits.  Gumming 
in  the  cherry,  peach,  plum,  or  apricot  is  even  more  common  than  in  the 
orange  or  lemon. 

Causes  of  Gumming. — The  production  of  gum  in  these  trees  has  been 
held  by  some  authorities  to  be  always  caused  by  mechanical  injuries. 
Cases  where  gumming  occurs  and  no  injury  can  be  found  they  attribute 
to  some  former  injury,  the  effects  of  which  become  evident  only  after 
the  wound  itself  has  disappeared,  or  to  an  injury  situated  at  some  dis- 
tance from  the  point  where  the  gumming  occurs.  It  is  evident  that 
injuries  often  do  start  gumming  and  that  this  is  greatly  increased  by 
a  plentiful  water  supply  at  the  roots;  but  that  the  latter  in  excess,  or 
some  other  physiological  disturbance,  may  actually  cause  as  well  as 
accelerate  gumming  is  not  so  generally  recognized.  That  such  is  the 
case,  however,  can  scarcely  be  doubted  from  California  experience. 
Many  instances  occur  and  some  fairly  characteristic  diseases  in  which 
violent  disturbances  in  the  physiological  processes,  as  well  as  some  in 
which  these  changes  are  not  violent  but  extremely  obscure,  result  in 
serious  injury  to  the  vitality  of  the  plant,  accompanied  by  various  forms 
of  gumming  from  the  affected  parts. 

In  the  sour-sap  disease  of  stone  fruits,  for  instance,  supposedly  caused 
by  a  sudden  change  from  vigorous,  unseasonably  early  activity  to  a 
period  of  dormancy  produced  by  the  occurrence  of  cold,  rainy  weather, 
abundant  gumming  occurs  on  the  affected  trees  or  branches.  Gum 
pockets  are  often  formed  on  cherry  trees  in  wet  spring  weather  on 
sound,  uninjured  branches.  In  the  peach  profuse  gumming  occurs 
in  wet  weather  on  twigs  affected  by  the  blight  fungus  (Coryneum 
beyerinkii) ,  but  in  the  same  tree  gumming  is  a  common  effect  when 
injured  by  unseasonably  warm  winter  weather  or  other  effects  not  of 
a  parasitic  or  mechanical  nature.  That  the  same  is  true  of  citrus  trees 
admits  of  no  doubt.  Gumming  is  an  indication  of  something  wrong. 
The  effect  is  distinctly  pathological  and  not  normal.  It  takes  many 
different  forms  and  some  very  characteristic  ones,  varying  with  the 
species  of  the  tree  and  the  conditions  which  cause  the  trouble.  The 
disease  does  not,  however,  require  the  occurrence  of  a  parasitic  organism 
or  mechanical  injury  to  account  for  its  appearance.  Nor  can  cause  and 
effect  always  be  closely  identified,  for  the  place  on  the  tree  and  the 
time  of  gum  formation  may  show  no  evident  connection  with  the  cause, 
and  the  cause  itself  may  be  very  obscure,  since  these  trees  are  peculiarly 
liable  to  functional  derangements  in  which  the  exact  conditions  which 
produce  them  and  the  exact  manner  in  which  they  come  about  are  most 
difficult  of  determination. 


240  UNIVERSITY  OF   CALIFORNIA— ^EXPERIMENT   STATION. 

Place  of  Gumming. — The  point  where  the  gumming  breaks  out  in  these 
eases,  where  no  infection  by  a  parasite  and  no  mechanical  injury  is 
involved  as  the  original  cause,  depends  entirely  on  local  conditions  in 
the  tree,  its  specific  peculiarities,  the  condition  of  its  tissues,  and  other 
individual  factors.  If  the  exciting  cause  is  one  of  climate,  soil,  or  mois- 
ture conditions,  usually  the  whole  tree  is  equally  affected.  With  the 
tendency  to  gumming  excited,  the  actual  outbreak  occurs  at  the  point  of 
least  resistance,  which  may  be  near  an  injury,  recent  or  old,  at  a  place 
having  relation  to  the  point  of  budding  or  of  the  giving-off  of  branches, 
or  at  places  in  the  tree  where  for  some  reason  the  structure  of  the  tissues 
is  in  a  condition  to  produce  irregularities  in  the  flow  of  sap  or  other  func- 
tions. The  case  is  almost  entirely  analogous  in  this  respect  to  the  con- 
dition of  a  person  affected  by  ulcers,  boils,  or  similar  outbreaks.  Back 
of  the  visible  effects  is  a  deranged  condition  of  some  vital  function  or 
organ,  the  result  of  which  shows  itself  as  a  superficial  eruption  or  other 
effect  at  the  point  on  the  body  which  for  some  local  cause  is  most 
susceptible. 

The  lesion  thus  produced  may  then  by  infection  or  processes  of  decay 
and  irritation  persist  and  increase,  even  after  the  original  causative 
condition  has  disappeared.  A  similar  conception  is  justified  in  regard 
to  the  various  peculiarities  in  the  visible  appearance  of  gum  disease. 
The  lesions,  ulcers,  or  affected  areas  produced  are  not  primarily  the 
seat  of  the  trouble.  They  represent  rather  the  effect  of  what  may  be 
called  a  general  constitutional  derangement  showing  itself  by  external 
outbreaks  or  symptoms  at  whatever  points  may  chance  to  be  most 
susceptible. 

Physiological  Diseases. — That  such  constitutional  derangements  pro- 
ducing the  so-called  physiological  or  autogenous  diseases  occur  in  plants 
the  same  as  in  animals  and  even  without  the  occurrence  of  any  radically 
unfavorable  or  injurious  conditions  to  account  for  them,  is  evidenced 
by  such  troubles  as  Beet  "Curly  Top,"  "California  Vine  Disease,"  and 
the  Aster  "Yellows."  In  all  these  cases  what  may  be  called  a  primary 
weakness  exists  back  of  the  visible  symptoms,  and  in  this  weakness  can 
be  sought  the  fundamental  cause  and  nature  of  the  disease.  It  is,  there- 
fore, not  necessary  to  identify  a  parasite  or  strikingly  evident  climatic 
or  soil  conditions  to  account  for  diseases  of  this  class. 

All  Trees  Not  Affected.— The  question  is  often  asked,  Why,  if  climatic, 
cultural  or  similar  conditions  of  some  sort  produce  the  ordinary  forms 
of  citrus  gum  disease,  all  trees  in  the  grove  growing  under  practically 
identical  conditions  are  not  similarly  affected?  Why  is  one  tree  com- 
pletely covered  with  scaly  bark  and  those  on  every  side  of  it  entirely 
dree  from  the  disease?     This  is  explained  by  the  same  reason  that  all 


Bulletin  200.  GUM  DISEASE  OF  CITRUS  TREES.  241 

the  trees  in  the  peach  or  apricot  orchard  do  not  take  sour-sap  alike  or 
all  the  beets  in  the  field  have  curly  top.  Also  that  all  men  do  not  take 
cold  or  indigestion  when  exposed  to  exactly  similar  conditions.  Trees 
like  persons  have  individuality.  No  two  are  in  exactly  the  same  condi- 
tion or  respond  exactly  alike  to  the  same  influences.  The  exciting  cause 
tending  to  produce  gum  disease  may  act  exactly  alike  upon  all  the 
trees  in  the  orchard,  but  affect  them  in  various  degrees.  Some  may  be 
not  at  all  affected,  some  influenced  to  a  certain  extent,  but  finally  pass 
the  critical  stage  without  visible  effect,  while  in  others  the  influence  is 
too  great  and  external  outbreaks  occur. 

In  citrus  trees  gumming,  on  the  whole,  has  more  characteristic  and 
definite  forms  than  in  the  stone  fruits.  For  this  reason  diseases  of 
this  nature,  which  are  abundant  wherever  citrus  trees  are  grown, 
have  received  various  distinguishing  names  according  to  the  various 
manifestations  of  the  trouble.  An  examination  of  the  literature  of 
citrus  culture  shows  that  in  every  citrus  growing  district  of  the  world 
gum  disease  plays  a  prominent  part,  though  the  form  taken  by  the 
malady  varies  to  a  considerable  extent. 


GUM  DISEASE  IN   CALIFORNIA. 

Under  the  general  term  "gum  disease,"  as  comprehended  in  this 
bulletin,  we  include  all  the  more  definite  troubles  with  citrus  trees  in 
California  in  which  the  exudation  of  gum  takes  place. 

Gum  Disease  Not  Infectious. — It  may  be  said  here  without  extended 
discussion  that  in  no  case  have  we  been  able  to  recognize  or  demonstrate 
the  presence  of  any  fungus,  bacterium,  or  other  parasitic  organism  as 
the  cause  of  any  form  of  citrus  gum  disease.  This  phase  of  the  matter 
has  received  very  careful  attention  at  the  Whittier  laboratory  during 
the  past  three  years,  and  from  the  very  numerous  cultures  and  micro- 
scopic examinations  which  have  been  made  without  finding  any  indica- 
tion whatever  of  any  such  parasite,  we  feel  safe  in  concluding  that  the 
diseases  hereafter  described  are  of  a  physiological  or  autogenous  nature, 
brought  about  by  a  deranged  condition  of  the  tree  itself.  This  con- 
clusion is  based  on  a  very  large  amount  of  careful  work,  and  coincides 
with  the  results  of  much  observation  in  the  field  as  to  the  probable 
nature  of  these  diseases,  as  well  as  the  experience  of  many  practical 
growers.  The  appearance  and  occurrence  of  the  diseases  is  not  such  as 
to  point  to  parasitic  infection  as  the  primary  cause  when  the  matter  is 
closely  followed  up. 


242  UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION. 

GUMMING  FROM  INJURIES. 

The  tendency  of  citrus  trees,  as  well  as  those  of  the  stone  fruits,  to 
form  gum  when  mechanically  injured  is  readily  demonstrated  by  experi- 
ment. If  the  trunk  of  a  citrus  tree  is  injured  in  almost  any  manner, 
either  by  bruising,  pounding,  or  by  the  application  of  acid  or  any 
injurious  substance,  gumming  frequently  results.  The  flow  of  gum  may 
not  be  confined  entirely  to  the  point  where  the  wound  is  made,  but  often 
an  irritation  is  produced  which  is  transmitted  to  a  considerable  distance 
and  the  gumming  spreads  up  the  trunk.  Cases  are  not  infrequent  in 
fumigation  where  sulphuric  acid  or  the  residue  from  the  combined  acid 
and  cyanide  of  potash  is  poured  against  the  trunk  of  the  tree  or  upon 
the  main  roots,  and  the  side  of  the  trunk  up  to  a  considerable  height, 
far  above  the  point  where  the  liquid  was  applied,  is  injured  and  breaks 
out  in  gumming.  No  definite  disease  can  be  described  as  a  result  of 
injury,  but  the  gumming  produced  in  this  way  is  well  known  and  very 
suggestive. 

LEMON   GUM  DISEASE  OR  GUMMOSIS. 

The  term  Gummosis,  if  it  be  applied  to  any  one  specific  trouble,  may 
be  best  given  to  the  disease  now  under  consideration.  While  commonly 
distinguished  as  the  gum  disease  of  the  lemon,  the  characteristic  trouble 
is  not  confined  to  lemon  trees,  and  therefore  a  name  which  suggests  this 
limitation  is  not  well  chosen..  "We  therefore  suggest  the  specific  use  of 
the  name  "Gummosis"  for  reference  to  this  form  of  disease.  Gummosis 
is  characterized  by  a  breaking  out  of  gummy  sap  on  the  trunk  of  the 
tree,  from  the  point  of  budding  up  to  a  rather  limited  height.  The 
exudation  of  gum  commonly  stops  abruptly  at  the  line  showing  the  place 
where  the  tree  was  budded.  It  has  little  tendency  to  run  down 
into  the  roots,  nor  does  it  in  typical  cases  spread  up  into  the  branches 
or  even  high  on  the  trunk.  In  the  most  characteristic  form  of  the  disease 
the  breaking  out  of  gum  takes  place  for  only  a  short  distance  above  the 
crown  of  the  tree. 

Not  Florida  Foot-rot. — In  two  respects  the  disease  differs  decidedly 
from  the  usual  foot-rot  of  Florida  and  Europe;  it  does  not  affect  the 
root  or  cause  a  decay  of  the  bark. 

Closer  examination  shows  that  the  gum  does  not  originate  in  the  bark, 
but  breaks  out  through  the  latter  by  mechanical  pressure  from  within. 
If  the  bark  is  removed  from  the  affected  area,  it  is  found  to  be  separated 
from  the  wood  to  a  considerable  extent,  and  the  cambium  layer  just 
between  the  bark  and  wood,  as  well  as  the  inner  bark  and  outer  por- 
tions of  the  wood,  is  discolored  and  more  or  less  affected.  The  area 
affected  may  be  large  or  small,  according  to  conditions,  or  may  continue 


Bulletin  200. 


GUM    DISEASE   OF    CITRUS   TREES. 


243 


to  spread  around  the  trunk  until  the  tree  is  completely  girdled.  In 
severe  cases  the  bark  of  the  affected  portions  dies  and  the  tree  itself 
may  do  the  same. 

In  this  form  of  disease  there  is  little  or  no  indication  of  any  abnormal 
growth  of  the  bark,  such  as  scabbing  or  scaling,  but  simply  a  breaking 
through  of  the  gummy  sap  from  within  by  mechanical  pressure.     An- 


Fig.  2.     Gummosis  of  lemon. 


other  characteristic  effect  of  this  trouble  is  an  abnormal  setting  of  the 
fruit  by  affected  trees,  together  with  premature  maturity.  Trees  badly 
affected  are  always  covered  with  a  large  amount  of  prematurely  ripe 
fruit,  showing  in  lemons  a  bright  yellow  color  while  still  small.  Gum- 
mosis has  been  much  more  prevalent  in  wet  years  than  those  in  which 
the  winter  rains  were  in  small  amount.    This  relation  of  a  large  amount 


244  UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION. 

of  soil  moisture  is  prominent  in  every  way,  both  as  regards  rainfall  and 
irrigation. 

The  effects  of  gummosis  in  the  orchard  have  been  very  severe  in  many 
groves,  particularly  with  lemons.  Many  trees  have  died,  and  in  some 
orchards  a  large  portion  of  the  acreage  has  been  lost  or  severely  injured. 

Relation  to  Soil  and  Location. — The  occurrence  of  this  form  of  gum 
disease  is  very  manifestly  connected  with  certain  conditions  of  soil  and 
location.  It  is  almost  invariably  seen  in  trees  growing  in  poorly  drained 
situations,  at  the  lower  end  of  orchards  situated  on  a  slope,  in  places 
where  water  frequently  stands  about  the  trees,  in  heavy  soils  or  in  trees 
which  have  become  too  deeply  buried  by  washing  down  of  soil  in  irriga- 
tion.   Most  commonly  the  disease  comes  about  from  improper  conditions 


Fig.   3.     Lemon  grove,  with  portion  in  foreground  destroyed  by  gummosis. 

regarding  soil  moisture,  yet  this  can  not  be  made  an  absolute  rule. 
When  such  is  not  the  case,  however,  it  is  almost  invariably  true  that 
the  trees  are  too  deeply  buried,  the  trunk  above  the  point  of  bud  union 
being  surrounded  with  earth.  In  cases  where  this  earth  is  of  a  clayey 
or  heavy  nature,  packing  closely  about  the  trunk,  and  particularly,  as 
is  the  usual  case,  if  it  is  not  disturbed  by  cultivation,  but  allowed  to 
become  hard  and  dry,  the  condition  is  aggravated  and  the  liability  of 
gummosis  becomes  greater. 

The  conditions  enumerated  are  most  apt  to  occur  at  the  lower  end 
of  an  orchard  where  the  water,  together  with  the  soil  and  silt  which 
it  carries,  tends  to  accumulate.  For  this  reason,  as  stated  above,  the 
worst  cases  of  gummosis  and  the  majority  of  affected  trees  in  orchards 
situated  on  a  slope  are  found  at  the  lower  end.  Occasional  instances 
occur  where  the  disease  seems  to  develop  in  light  gravelly  soils  or  in 


Bulletin  200.  GUM  DISEASE  OF  CITRUS  TREES.  245 

places  other  than  the  lower  end  of  a  slope,  but  such  cases  are  due  very 
largely  to  a  surface  deposit  or  wash  of  gravel  overlying  a  hard,  heavy 
soil.  In  the  case  of  a  leaky  ditch  or  flume  running  through  or  by  the 
orchard  the  trees  in  that  vicinity  are  the  most  likely  to  be  affected 
with  gummosis, 

A  form  of  gum  disease  similar  to  this  is  also  easily  produced  by 
heaping  material  such  as  manure,  weeds,  or  green  cover  crop  close 
about  the  trunk  of  the  tree.  Occasionally,  in  applying  such  material, 
it  becomes  piled  about  the  trunk,  and  this  often  leads  to  gummosis. 

This  description  of  the  character  and  occurrence  of  gummosis,  or 
lemon  gum  disease,  will  be  sufficient  to  give  a  correct  idea  of  the  form 
of  disease  which  we  mean  to  include  under  this  heading.  A  description 
of  its  more  intimate  nature,  together  with  consideration  of  means  of 
relief  and  control,  may  now  be  taken  up. 


EFFECT  OF  GUMMOSIS  UPON  THE  TISSUES. 

The  flow  of  gum  from  the  trunk  of  trees  affected  with  this  disease, 
occurring  as  it  does  most  abundantly  under  conditions  of  excessive 
moisture,  is  often  thought  to  be  simply  a  breaking  out  of  the  sap  by 
mechanical  pressure  on  account  of  its  excessive  amount.  This,  however, 
is  not  strictly  the  case.  As  has  been  previously  mentioned,  the  gum 
itself  is  a  pathological  product  and  not  a  normal  constituent  of  the  sap. 
A  microscopic  study  of  the  condition  of  affected  tissues  has  been  made 
during  this  investigation,  and  while  it  need  not  be  reported  on  at  length 
in  a  bulletin  of  this  character,  something  may  be  said  as  to  the  real 
nature  of  the  process  of  gum  formation  and  exudation.  In  our  brief 
description  of  the  anatomy  and  physiology  of  the  citrus  tree  we  have 
called  attention  to  the  structure  of  the  trunk  and  mode  of  action  of 
the  sap  in  the  same.  We  pointed  out  that  in  the  outer  part  of  the  wood 
a  vigorous  flow  of  water  is  passing  upwards  to  the  leaves.  This  water 
also  is  absorbed  laterally  to  some  extent  sufficient  to  keep  the  trunk  and 
bark  full  of  moisture.  Meantime,  the  prepared  food  material  is  coming 
down  through  the  inner  bark  and  much  of  it  passing  to  the  cambium 
layer,  where  it  is  used  in  forming  new  tissue.  On  the  inner  side  the 
cambium  normally  keeps  laying  down  new  layers  of  wood  so  long  as 
growth  is  active.  In  other  words,  there  is  a  constant  deposit  of  new 
wood  on  the  outside  of  that  already  formed.  This  is  accomplished  by  the 
constant  transformation  of  the  cambium  layer  cells  into  wood  cells,  by  a 
deposit  of  thickening  layers  of  cellulose  upon  the  originally  thin  and 
delicate  walls.  It  is  just  in  this  process  that  the  actual  gum  formation 
takes  place.  The  material  which  ought  to  go  to  form  the  thickening  of 
the  wood  cells  stops  short  of  this  change  and  turns  into  gum,  which  is 


246 


UNIVERSITY   OF   CALIFORNIA EXPERIMENT   STATION. 


FlO.  4.  Microscopic  view  of  tissue  affected  by 
gum  disease;  an  enlargement  of  area  G 
in  figure  l.     W  norma]  wood,  PP  regions 

of  imperfectly  formed  wood,  cell  \v;ills 
not  thickened;  C  cambium,  G  sum  pockel 
in   new  wood,  .just   inside  cambium. 


thus  in  its  chemical  nature 
an  unformed  or  soluble 
form  of  what  should  be  the 
solid  plant  tissue.  This 
gum  accumulates,  forming 
a  pocket  just  under  the 
cambium,  or  soon  by  pres- 
sure occupying  the  space 
between  the  bark  and  wood. 
Water  also  accumulates  in 
this  pocket,  and  finally,  if 
the  process  continues,  suffi- 
cient pressure  is  set  up  to 
rupture  the  bark  and  the 
gum  escapes  to  the  outside. 
The  amount  of  gum  de- 
pends upon  the  extent  to 
which  the  changes  described 
take  place,  as  well  as  upon 
the  amount  of  water  in  the 
tree.  In  severe  cases  large 
areas  of  the  new  wood  are 
affected  at  many  different 
points,  and  thus  large  or 
numerous  gum  pockets  are 
formed,  which  results  in 
separating  a  large  area  of 
bark  from  the  wood.  After 
the  wound  has  thus  been 
formed,  infection,  fermen- 
tation, or  decay  may  set  in, 
causing  further  injury  to 
the  tree,  or  an  irritation 
may  be  produced  by  these 
factors,  and  the  efforts  of 
the  tree  to  heal  over  the 
lesion  may  result  in  a  con- 
tinuation of  the  trouble 
after  the  original  influence 
which  caused  the  com- 
mencement of  gumming  has 
passed  away. 

This,  in  brief,  describes 
the  nature  of  the  process  of 
gum    formation.      It    is   in 


Bulletin  200.  GUM  DISEASE  OF  CITRUS  TREES.  247 

many  respects  similar  to  the  occurrence  of  an  ulcer  or  boil  in  the 
human  race,  consisting  in  a  failure  of  the  formation  of  proper  tissue 
at  the  affected  point  and  the  secondary  effects  of  infection,  putrefaction 
or  other  irritation. 

METHODS  OF  CONTROL  OF   GUMMOSIS. 

Avoid  Unfavorable  Conditions. — The  most  important  consideration  in 
regard  to  the  control  of  gummosis  is  the  fact  that  since  the  disease  is 
brought  about  solely  by  unfavorable  conditions  it  can  be  more  easily 
prevented  by  avoiding  those  conditions  than  it  can  be  cured  after  once 
started.  By  the  same  reasoning,  the  cure  of  the  disease  lies  chiefly 
along  the  line  of  improvement  of  the  conditions  which  have  produced 
it  rather  than  by  any  treatment  of  the  tree  itself.  The  avoidance  of 
gum  disease  should  be  one  of  the  chief  considerations  of  the  grower 
starting  a  citrus  grove,  particularly  with  lemons.  In  the  choice  of 
location  heavy,  wet  ground  should  be  avoided,  unless  it  can  be  drained 
or  improved  in  some  manner. 

Sour  Orange  Boot. — On  land  which  is  otherwise  desirable  for  plant- 
ing, but  has  some  qualities  which  would  make  it  liable  to  gum  disease, 
considerable  immunity  can  be  obtained  by  planting  trees  budded  upon 
the  Florida  sour-stock  root,  "We  have  made  careful  inquiry  as  to  the 
success  of  trees  upon  this  root  in  California,  and  find  that  there  is 
ample  experience  to  justify  its  use  in  heavy  soils.  The  root  has  no 
advantage  over  the  ordinary  sweet  seedling,  except  for  the  much  greater 
freedom  from  gum  disease  which  it  imparts.  Trees  grown  on  the  sour- 
stock  are  rather  slow  of  development  during  the  first  few  years,  but 
bear  abundant  crops  of  fruit,  and  as  time  goes  on  they  overtake  those 
of  the  same  age  on  sweet-stock,  so  that  in  mature  trees  there  is  prac- 
tically no  difference  on  the  two  roots.  We  strongly  recommend  the  use 
of  the  sour-stock  root  for  all  citrus  trees  to  be  planted  on  heavy  soil 
which  is  liable  to  gum  disease. 

High  Budding. — Another  important  consideration  for  avoiding  gum 
disease  is  high  budding  of  the  nursery  stock.  As  we  have  previously 
shown,  trees  which  are  affected  in  the  orchard  usually  have  the  point 
of  budding  deeply  buried  in  the  soil,  and  this  condition  contributes 
greatly  to  the  disease.  This  may  be  easily  avoided  by  budding  the  trees 
a  foot  from  the  ground,  a  practice  which  has  no  disadvantage  except 
the  somewhat  longer  time  required  to  produce  such  a  tree.  Possibly 
an  extra  season  may  be  necessary  to  get  the  young  seedlings  up  to 
sufficient  size  to  bud  them  at  the  desired  height.  The  advantage  gained, 
however,  is  much  greater  than  this  slight  objection.  Whether  the  trees 
are  high-budded  or  not,  care  should  be  taken  not  to  plant  them  too 


248 


UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION. 


deeply  in  the  ground.     Gum  disease  is  induced  in  a  great  many  young 
trees  the  first  year  in  the  orchard  by  planting  them  with  the  bud  union 


Fig.  5.     High  budded  lemon  tree  ;  desirable  to  avoid  gum  disease. 

below  the  surface,  where  they  become  covered  with  soil.     This  is  ex- 
tremely undesirable. 


Bulletin  200.  GUM  disease  OF  CITRUS  TREES.  249 

Cultural  Prevention. — The  prevention  of  gummosis  in  the  orchard  is 
entirely  a  matter  of  the  avoidance  of  the  conditions  which  have  been 
shown  to  be  the  cause  of  the  disease.  The  accumulation  of  soil  or  water 
about  the  trunks,  allowing  the  same  soil  to  become  hardened,  lack  of 
drainage  or  cultivation,  or  the  accumulation  of  surface  water,  either 
from  irrigation  or  rain,  must  be  carefully  avoided.  All  these  precau- 
tions are  particularly  necessary  in  places  where  the  soil  conditions  are 
such  as  to  favor  the  disease.  In  such  places  special  care  must  be  taken 
to  keep  the  ground  thoroughly  stirred  close  up  about  the  trunk  of  the 
trees  and  prevent  either  soil  or  water  from  accumulating  about  them. 
If  this  is  accomplished  one  need  have  little  fear  of  gummosis. 

Improvement  of  Soil  Conditions. — The  treatment  of  affected  trees 
must  consist  first  of  all  in  an  improvement  of  soil  conditions.  It  is  idle 
to  work  upon  the  trees  themselves  or  to  look  for  any  remedy  for 
application  to  the  affected  parts  until  the  original  conditions  which 
produce  the  trouble  have  been  corrected.  If  this  is  not  done,  the  work 
done  upon  the  trees  will  be  thrown  away,  since  gummosis  will  begin 
again  at  the  first  favorable  time.  In  treatment  of  affected  trees  one 
should  therefore  begin  with  a  thorough  stirring  of  the  soil  about  the 
trees  clear  up  to  the  trunks.  It  will,  no  doubt,  be  necessary  to  do  much 
of  this  by  hand  with  a  mattock,  but  there  is  no  other  method  of 
accomplishing  the  desired  result.  After  thoroughly  loosening  up  the 
soil  a  space  of  at  least  two  feet  in  diameter  must  be  cleared  about  the 
trunk,  removing  the  accumulated  soil  down  to  the  roots,  exposing  the 
bud  union.  In  many  cases  a  foot  or  more  of  washed-in  soil  will  be 
found  at  this  point. 

Cutting  the  Bark. — After  this  has  been  done  work  may  commence 
upon  the  tree  itself  to  improve  its  condition.  The  bark  of  the  tree 
where  gumming  has  occurred  may  be  cut  out  to  some  extent  to  relieve 
the  pressure  and  permit  the  escape  of  the  accumulated  gum.  In  some 
cases  the  whole  bark  has  been  stripped  off  over  all  the  affected  portion, 
while  in  others  only  narrow  strips  of  bark  have  been  taken  out,  running 
from  the  crown  up  to  the  main  fork  on  several  sides  of  the  tree.  The 
latter  process,  on  the  whole,  is  thought  to  be  the  better  method. 

Stripping. — If  the  bark  is  stripped  off  entirely  a  large  area  is  exposed 
on  badly  affected  trees,  and  in  some  cases  it  is  necessary  to  entirely 
girdle  the  tree.  Under  improved  soil  conditions  much  of  this  bark 
would  be  likely  to  recover  and  remain  alive,  or  if  it  dies  it  serves  to 
protect  the  trees  and  cover  the  new  growth  of  bark,  which  often  comes 
beneath  it.  Many  cases,  however,  have  been  successfully  treated  by 
stripping  off  all  the  bark  which  showed  discoloration  beneath  it,  cutting 
out  cleanly  about  the  edges  and  painting  over  the  exposed  surface  with 


250 


UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION. 


some  protective  covering.  In  such  treatment  the  bark  should  be  peeled 
off  without  scraping  the  surface  of  the  wood,  as  often  much  of  the 
cambium  layer  is  still  alive  and  will  form  a  new  layer  of  bark. 

Treatment  of  Wounds.— For  covering  the  exposed  surface  a  form  of 
grafting  wax  has  been  found  most  satisfactory.     This  is  prepared  by 


4  y 
M 


:::-./,    ■    "/ 


Fig.  6. 


Lemon  tree  treated  for  guramosis  by  stripping  off 
affected  bark  and  covering  with  wax. 


melting  together  4  pounds  of  resin,  1  pound  of  beeswax,  and  1  pound 
of  raw  linseed  oil.  After  thoroughly  mixing,  the  wax  is  painted  on 
with  a  brush  while  still  warm  and  liquid.  In  many  cases  a  considerable 
amount  of  new  bark  can  be  seen  forming  under  this  transparent  wax, 
and  the  tree  makes  a  decided  recovery.  A  great  variety  of  other 
materials  have  been  used  for  covering  the  wound,  but  none  of  them 
appear  to  be  as  satisfactory  as  the  wax. 


Bulletin  200. 


GUM    DISEASE   OF    CITRUS    TREES. 


251 


Slitting. — The  method  of  slitting  the  bark,  taking  out  one-eighth  inch 
strips  on  four  or  five  sides  of  the  tree,  through  both  the  affected  and 
the  healthy  parts,  is  usually  most  advisable.  A  special  knife  can  be 
made  for  this  work  with  which  the  slitting  can  be  done  quite  rapidly. 


Fig.  7.  Lemon  tree  treated  for  gummosis  by  slitting-  bark.  Two 
whitest  lines  are  new  slits.  Others  are  older  ones.  Note 
excavation    about   trunk. 

Surface  Applications. — After  cutting  the  bark  in  this  manner  some 
application  may  be  made  to  the  surface,  and  for  this  purpose  many 
different  substances  have  been  tried.  The  application  of  neat's-foot 
oil,  recently  recommended  by  Judge  A.   P.   Call,  of  Corona,  appears 


252 


UNIVERSITY   OF   CALIFORNIA EXPERIMENT   STATION. 


quite  effective,  the  oil  tending  to  soften  the  bark  and  gum  and  keeping 
the  latter  from  collecting  in  hard  masses  on  the  bark.  A  one-tenth 
solution  of  caustic  potash  in  water  has  also  been  used  with  good  results. 
Others  have  used  crude  carbolic  acid,  sheep  dip,  and  a  great  variety 


Fig. 


Lemon  tree  pruned  back  after  treatment  for  gummosis. 
excavation  about  trunk. 


Note 


of  other  materials.  The  liquid  substance  recommended  should  be 
painted  over  the  whole  trunk  of  the  tree,  making  a  thorough  applica- 
tion. 

Soil  Improvement  Most  Important. — The  whole  matter  of  the  treat- 
ment of  the  tree  itself  is  entirely  of  secondary  importance.  If  proper 
soil  conditions  can  be  obtained  and  maintained,  the  tree  will  heal  itself, 
if  not  too  far  gone.  The  only  benefit  to  be  derived  from  any  method  of 
bark  cutting  and  surface  application  is  to  promote  the  healing  of  the 
wounds  caused  by  the  disease. 


Bulletin  200. 


GUM    DISEASE   OF    CITRUS    TREES. 


253 


Pruning. — After  treating  the  trunk  of  the  tree  it  is  well  to  cut  back 
the  top  to  some  extent,  reducing  the  amount  of  branches  in  proportion 
to  the  amount  of  bark  lost  from  the  trunk. 

After  the  treatment  of  the  soil  and  trees  has  been  accomplished, 
measures  must  be  taken  to  prevent  as  far  as  possible  the  recurrence  of 


Fig.   9.     Lemon  tree  boxed  at  base  after  excavation  and  slitting  bark. 

the  conditions  which  brought  about  the  trouble.  When  possible  to  do 
so,  the  general  level  of  the  soil  about  the  tree  should  be  cut  down  so 
that  the  point  of  bud  union  will  be  well  above  ground. 

Use  of  Sand  About  Trunk. — If  it  is  impossible  to  do  this,  the  hole 
which  was  dug  out  about  the  trunk  must  be  kept  open  so  that  the  soil  will 


254  UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION. 

not  accumulate  again  at  that  point,  and  at  the  same  time  this  hole  must 
not  be  allowed  to  become  a  receptacle  for  irrigation  and  rain  water, 
thus  making  the  condition  worse  than  ever.  In  such  cases  the  practice 
of  filling  the  space  with  coarse  sand  has  been  resorted  to  quite  success- 
fully, thus  preventing  the  accumulation  of  water  to  quite  an  extent, 
and  at  the  same  time  not  producing  a  layer  of  hard  soil  about  the  trunk. 

Boxing. — Another  method  consists  in  making  a  wooden  box  from  one 
to  two  feet  square  about  the  tree  with  the  sides  high  enough  to  keep 
out  surface  water.  This  is  somewhat  expensive,  but  is  probably  the 
best  method  that  can  be  suggested  for  permanent  improvement  in  cases 
where  the  trees  are  too  low.  In  places  where  surface  water  accumulates, 
temporary  open  ditches  may  often  be  made  to  advantage  during  the 
winter  to  carry  off  the  surplus. 

Without  further  details,  we  will  say,  in  general,  that  the  whole  pre- 
vention of  gummosis  consists  in  good  drainage  and  proper  aeration  of 
the  roots. ' 

The  following  letter,  recently  written  by  Judge  A.  F.  Call,  of  Corona, 
is  well  worth  reproduction  in  this  connection : 

"I  found  on  coming  to  California,  early  last  year,  that  I  had  twelve  hundred 
lemon  trees  out  of  three  thousand  more  or  less  affected  with  gum,  and  four 
hundred  orange  trees  out  of  sixteen  thousand  affected  with  so-called  scab.  After 
treatment  for  one  year,  all  the  gummed  lemon  trees,  with  the  exception  of  four,  have 
either  fully  recovered,  or  are  well  on  their  way  to  recovery,  and  now  of  a  deep 
green  color,  well  set  with  fruit,  and  all  of  the  scabbed  orange  trees  are  showing 
marked  improvement  and  on  the  way  to  recovery.  I  have  made  it  a  point  to 
investigate  these  troubles  in  nearly  every  colony  in  California  and  several  districts 
in  Florida,  and  believe  that  I  know  the  cause  and  cure  for  this  trouble,  and  am 
satisfied  that  if  drainage  conditions  will  permit,  these  troubles  can  be  prevented 
and  trees  that  are  not  entirely  gone  can  be  cured. 

"I  came  to  the  conclusion  that  this  trouble  was  not  a  disease,  but  was  simply 
a  mechanical  proposition,  and  to  demonstrate  this  I  inoculated  several  healthy 
trees  by  inserting  chunks  of  gum  under  the  bark,  with  the  result  that  no  tree  was 
affected.  On  the  other  hand  any  injury  to  the  bark  will  cause  the  gum  to  exude. 
This  may  be  caused  in  a  variety  of  ways.  In  some  instances  I  found  a  small 
root  encircling  the  trunk  of  the  tree  just  above  the  crown  roots,  which  had  become 
imbedded  in  the  bark  and  caused  a  stoppage  of  the  flow  of  sap.  In  other  cases  I 
found  that  hard  ground  had  buried  some  pebbles  in  the  bark,  with  the  same  result. 
In  other  cases  I  found  that  hot  manure  around  the  trunk  of  the  tree  deteriorated 
the  bark.  In  other  cases,  standing  water  around  the  trunk  of  the  tree  had  injured 
the  life  of  the  bark,  but  by  far  the  most  common  cause,  and,  in  fact,  perhaps  ninety 
per  cent  of  all  the  cases,  is  caused  by  earth  around  the  trunk  of  the  trees  above  the 
crown  roots.  As  I  look  at  it,  the  reason  of  this  accumulation  of  earth  affecting 
the  bark  is  that  the  bark,  which  nature  intended  to  be  above  the  ground,  is  more 
tender  than  the  bark  on  the  root,  which  nature  intended  to  be  below  ground,  and 
becomes  easily  affected,  when  deprived  of  air  and  in  contact  with  the  earth,  espe- 
cially wet  earth. 

"The  gum  disease  of  the  lemon  trees  and  the  scab  disease  of  the  orange  trees 
is  substantially  the  same  trouble,  and  comes  from  the  same  cause,  and  that  is  an 
iu jury  to  or  deterioration  of  the  bark  of  the  tree  which  impedes  the  downward  flow 
of  sup.  I  believe  an  examination  will  show  a  darkened  or  deteriorated  condition 
of  the  bark  some  where  on  the  trunk  of  the  tree  below  the  point  where  the  gum 
is  exuding. 


Bulletin  200.  GUm  DISEASE  OF  CITRUS  TREES.  255 

''The  treatment  should  commence  with  the  removal  of  the  cause,  and  that  is  by 
removing  the  earth  down  to  the  crown  roots  and  giving  the  tree  proper  drainage. 
This  should  be  done  very  carefully,  as  there  is  danger  that  the  hoe  or  shovel  of  the 
workman  will  bruise  or  cut  the  crown  roots  and  thus  cause  additional  trouble.  When 
the  earth  has  been  removed,  that  portion  of  the  trunk  which  has  been  covered  with 
earth  should  be  whitewashed,  to  prevent  injury  caused  by  sudden  exposure  to  the  air 
and  sun.  The  next  thing  to  do  is  to  split  the  bark  to  permit  the  escape  of  the 
surplus  sap,  to  prevent  its  accumulation  and  consequent  destruction  of  the  bark. 
This  should  be  done  by  making  a  narrow  furrow  from  one  sixteenth  to  one  eighth 
of  an  inch  in  width  from  the  limbs  to  the  ground.  About  four  of  these  furrows  are 
usually  sufficient  at  about  equal  distances  apart. 

"It  is  a  great  mistake  to  make  several  short  gashes  around  the  bud,  as  is 
frequently  done,  for  this  brings  the  surplus  sap  all  to  one  point  and  causes  a 
greater  injury.  It  is  also  a  mistake  to  remove  the  bark,  as  is  frequently  done,  which 
is  supposed  to  be  affected,  for  this  causes  a  big  wound  which  it  takes  a  long  time 
to  cure,  and  my  experience  has  shown  that  the  largest  part  of  the  bark  which 
appears  to  be  affected  will  recover  when  the  cause  is  removed  and  the  surplus 
sap  permitted  to  escape.  After  this  is  done  all  the  affected  portions  and  furrows 
through  the  bark  should  be  thoroughly  covered  with  neat's-foot  oil.  There  is  a 
double  advantage  in  the  application  of  this  oil.  One  is,  that  it  has  a  softening  effect 
on  the  bark  and  aids  in  restoring  its  sap-carrying  power.  The  other  is,  that  it 
prevents  the  gum  from  sticking  to  and  hardening  on  the  bark  and  facilitates  the 
removal  of  the  gum,  which  will  flow  from  the  furrows. 

"After  the  trees  have  been  treated  they  should  be  carefully  watched,  and  two  or 
three  weeks  after  the  first  treatment  they  should  be  gone  over  again,  and  gum  removed 
from  the  furrows,  with  a  second  application  of  the  oil,  and  this  watchfulness  and 
care  should  be  continued  until  the  gum  has  ceased  to  form. 

"It  will  be  found  that  at  the  end  of  the  year  all  the  trees  not  badly  affected 
will  have  recovered,  and  those  which  have  been  affected  over  considerable  area 
will  show  some  affection  just  above  the  old  scars,  owing  to  the  injury  of  the  bark 
still  existing,  with  not  enough  new  growth  to  carry  all  the  sap.  Most  of  the  trees 
carrying  a  large  amount  of  sap  may  have  been  so  badly  affected  that  there  is  not 
sufficient  amount  of  bark  left  to  carry  so  much.  When  this  condition  is  found,  or, 
in  fact,  wherever  the  tree  is  more  than  half  girdled  or  more  than  half  of  the  bark 
is  seriously  affected  or  dead,  the  top  should  be  cut  back  in  proportion  to  the  injured 
bark — that  is,  if  one  half  of  the  bark  has  been  destroyed,  one  half  of  the  top  should 
be  removed.  If  only  a  strip  two  inches  wide  of  good  bark  remains,  but  a  very 
small  top  should  be  left. 

"Last  year  I  had  one  tree  with  a  trunk  about  six  inches  in  diameter,  that  only 
had  left  a  strip  of  bark  one  inch  in  width.  I  cut  the  top  of  this  tree  severely. 
Now  that  strip  is  three  inches  in  width,  the  top  has  grown  vigorously,  the  tree 
is  full  of  fruit. 

"I  believe  that  any  one  having  a  grove  showing  gum  should  not  wait  for  it  to 
develop,  but  should  prevent  it  by  carefully  removing  all  the  earth  to  the  crown 
roots,  and  most  growers  will  be  astonished  to  find  how  deep  their  trees  are  in  the 
ground.  This  is  partly  caused  by  original  deep  setting,  partially  by  plowing  the 
earth  to  the  trunk  of  the  tree  and  leaving  it  there,  and  partially  by  sediment  carried 
from  the  upper  end  of  the  grove  to  the  lower  and  there  stopped  by  a  dike  or  head 
ditch. 

"After  the  earth  has  been  removed,  it  is  important  to  prevent  a  basin  for  water 
being  formed,  and  therefore  the  excavation  should  not  extend  more  than  a  foot 
from  the  trunk  of  the  tree.  The  trees  that  are  very  deep  should  be  boxed  with  a  box 
about  two  feet  square,  extending  above  the  ground.  The  trees  that  are  not  so 
deep  should  be  filled  with  loose  sand  or  gravel,  as  this  will  permit  some  aeration 
and  will  not  cause  the  disease  that  a  hard  or  wet  earth  will  cause. 

"In  making  the  furrows  in  the  trees  I  had  a  little  tool  made  which  is  much  better 
than  a  knife,  by  taking  a  thin  bladed  blacksmith's  knife  and  having  the  blacksmith 
bend  it  in  the  middle  to  a  sharp  angle  and  then  sharpen  it.  It  will  cut  a  furrow 
about  one  sixteenth  of  an  inch  wide  and  avoid  the  danger  of  gashing  or  haggling  the 
tree,  which  might  be  done  by  a  careless  workman  with  a  knife." 


256 


UNIVERSITY  OF   CALIFORNIA — EXPERIMENT   STATION. 


SCALY  BARK,  OR  PSOROSIS. 

This  is  primarily  a  disease  of  the  orange.     While  its  occurrence  on 
other  citrus  trees  may  not  be  unknown,  it  is  certainly  extremely  rare  in 


Fig.  10.     Orange  tree  affected  with  scaly  bark. 

a  typical  form.  Scaly  bark  is  distinguished  particularly  from  gummosis 
by  its  occurrence  at  any  point  upon  the  branches  or  trunk  of  a  tree, 
with  no  apparent  direct  connection  between  the  soil  and  the  location 
of  the  affected  area.    Also  by  the  scaling  off  in  flakes  of  the  outer  bark, 


Bulletin  200. 


GUM    DISEASE    OF    CITRUS    TREES. 


257 


leaving  the  inner  layer  nearly  intact,  instead  of  a  separation  between 
wood  and  bark,  as  in  gummosis.  Most  commonly  the  disease  appears 
first  upon  the  trunk,  although  often  at  a  considerable  distance  from  the 
ground  in  large  trees,  but  it  commonly  breaks  out  in  isolated  spots  upon 
the  branches,  even  upon  those  of  quite  small  size. 

The  appearance  of  the  disease  is  suggested  by  its  popular  name.     At 


Fig.   11.     Scaly  bark  area  on  orange  limb. 

a  certain  point  on  the  trunk  or  branch  the  surface  layer  of  bark  begins 
to  break  out  in  scales  which  curl  up  and  separate  from  the  inner  bark. 
This  separation  of  the  bark  into  scales  does  not  occur  at  the  cambium 
layer  between  the  bark  and  wood,  but  involves  simply  the  outer  rough 
bark.  The  inner  bark  remains  in  its  proper  position  and  becomes  more 
or  less  pushed  out  as  the  disease  progresses,  owing  to  the  irregular 
growth  of  the  wood  beneath.     The  scaly  portion  is  usually  only  an  inch 


258  UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION. 

or  two  in  diameter  at  first,  but  gradually  spreads  until  it  covers  a  large 
patch,  or  may  involve  the  whole  surface  of  the  trunk  and  much  of  that 
of  the  limbs.  There  is  no  very  abundant  production  of  gum  from  the 
affected  portions,  but  here  and  there  a  small  pustule  breaks  out  from 
which  gum  exudes  of  a  similar  nature  to  that  seen  in  gummosis.  The 
gum  is  less  abundant  than  in  the  latter  disease.  Trees  affected  with 
scaly  bark  soon  lose  their  healthy  appearance  as  the  disease  develops, 
the  foliage  becomes  sparse  and  light  colored,  and  the  branches  die  back 
so  that  the  top  of  the  tree  becomes  full  of  dead  wood.  Such  trees, 
however,  seldom  die  completely,  but  linger  for  many  years,  throwing 
out  new  shoots  from  the  trunk  and  maintaining  a  feeble  existence. 
Scaly  bark  is  most  commonly  seen  on  old  trees  rather  than  young  ones, 
and  is  especially  common  in  some  of  our  old  seedling  orchards  or  the 
oldest  groves  of  the  Washington  Navel. 

Relation  to  Soil. — The  occurrence  of  this  disease  with  reference  to  any 
particular  condition  of  soil,  cultivation,  or  climate  is  rather  less  evident 
than  in  the  case  of  gummosis.  It  has  no  pronounced  relation  as  in  the 
latter  case  to  heavy  soils  or  excess  of  moisture,  or  at  least  not  in  the 
evident  manner  which  is  seen  in  the  other  diseases. 

Becomes  Chronic. — One  feature  of  scaly  bark  which  makes  the  deter- 
mination of  the  causes  which  bring  it  about  quite  difficult  is  the  fact 
that  in  the  majority  of  cases  affected  trees  have  been  in  their  present 
condition  for  a  great  many  years.  The  disease  shows  no  rapid  spread 
or  greatly  increased  prevalence,  but  so  far  as  can  be  learned  has  been 
in  much  the  same  condition  as  at  present  for  a  long  time.  Many  trees 
can  be  identified  which  have  been  affected  for  at  least  fifteen  years,  and 
while  their  appearance  is  very  bad,  it  would  seem  from  what  can  be 
learned  that  their  condition  has  changed  very  little  during  this  period. 
In  the  majority  of  cases  the  trees  in  an  orchard  affected  by  the  scaly 
bark  are  scattered  indiscriminately  about,  with  no  decided  relation  to 
each  other  or  to  any  particular  condition.* 

Belated  to  Irregular  Water  Supply. — So  far  as  can  be  learned  from 
a  large  amount  of  field  observation  it  would  seem  that  scaly  bark  is 
connected,  more  than  with  any  other  condition,  with  one  of  extreme 
changes  in  the  moisture  condition  of  the  soil.  In  other  words,  where 
the  soil,  together  with  the  roots  of  the  tree,  becomes  alternately  very 
wet  and  very  dry. 

This  is  brought  out  to  a  considerable  extent  by  a  microscopic  exam- 
ination of  the  wood  of  orange  trees  affected  by  scaly  bark.  In  ordinary 
deciduous  treas  the  growth  is  commonly  divided  into  two  periods  each 
year :  one  of  activity  and  one  of  rest.     During  the  growing  portion  of 


*  See  page  240. 


Bulletin  200.  GUM  DISEASE  OF  CITRUS  TREES.  259 

the  season,  that  is,  in  the  summer,  a  ring  of  wood  is  formed  and  the 
number  of  these  rings  corresponds  to  the  age  of  the  tree.  In  the  citrus 
tree  there  is  no  such  annual  growth,  but  a  more  or  less  continual  devel- 
opment all  the  year  -round.  The  wood  of  an  orange  tree  grown  under 
good  cultivation  with  a  uniform  condition  of  moisture,  never  too  wet 
or  too  dry,  has  a  very  uniform  structure  with  only  a  slight  indication 
of  ring  formation.  The  wood  of  a  scaly  bark  tree,  on  the  other  hand, 
always  shows  very  evident  rings,  corresponding  not  to  years  but  to 
periods  of  growth,  of  which  several  occur  during  the  same  year.  The 
more  irregular  the  conditions  of  growth  the  more  prominent  are  the 
rings,  and  in  neglected  trees  which  are  watered  irregularly  the  ring 
formation  is  more  marked,  compared  with  those  which  are  well  cared 
for.  This  being  particularly  true  in  scaly  bark  trees  shows  that  their 
growth  has  been  irregular  rather  than  uniform. 

If  it  be  asked  why  trees  which  are  uniformly  irrigated  still  show  scaly 
bark,  it  should  be  remembered  here  again  that  the  majority  of  those 
now  affected  have  been  in  this  condition  for  many  years,  and  the  con- 
ditions which  brought  about  the  disease  may  have  totally  changed.  The 
effect  on  the  tree  once  begun  may  remain  and  continue  to  spread  long 
after  the  original  cause  has  passed  away. 

In  a  number  of  pronounced  cases  of  scaly  bark  in  somewhat  neglected 
or  poorly-cared-for  orchards,  we  have  found  the  principal  area  of 
affected  trees  to  be  situated  near  the  flumes  or  source  of  water  supply. 
This  is  particularly  true  over  hardpan,  or  where  by  shallow  cultivation 
an  artificial  hardpan  has  been  formed.  In  such  places  the  soil  becomes 
very  wet  during  irrigation  and  very  dry  between  times.  To  this  con- 
dition may  be  ascribed  many  of  the  most  pronounced  cases  of  scaly  bark. 

Further  than  this  we  have  seen  orchards  containing  areas  of  this 
disease  where  no  improper  conditions  are  now  present,  but  where  it  is 
stated  that  in  former  times  the  water  was  accustomed  to  settle  or 
accumulate  during  the  irrigation  by  the  flooding  method  then  in  vogue. 
Other  cases  of  scaly  bark  in  individual  trees  have  seemed  to  show  con- 
nection with  a  deep  burying  of  the  roots,  the  planting  of  a  tree  over  a 
boulder  or  area  of  hard  soil,  or  some  other  circumstance  which  would 
bring  about  the  condition  described. 

It  is  our  opinion  that  a  large  majority  of  the  present  cases  of  disease, 
affecting  isolated,  scattering  trees  in  groves  where  no  unfavorable  con- 
ditions can  be  detected,  are  relics  of  a  former  period  of  cultural  or 
irrigation  practice  different  from  that  now  in  vogue.  The  reason  why 
individual  trees,  rather  than  the  whole  grove,  were  affected  has  already 
been  attributed  to  differences  in  individuality  and  possibly  also  to 
varying  soil  conditions  immediately  underlying  the  trees. 


260  UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION. 

Ah  Extreme  Case. — We  have  known  of  one  instance  where  scaly 
bark  was  brought  on  in  an  almost  epidemic  manner  by  a  radical  treat- 
ment of  an  orchard.  In  this  case,  of  some  large  seedlings,  the  grove  was 
"very  much  neglected  as  to  irrigation  and  cultivation  for  a  season,  and 
was  finally  pastured  with  cattle.  In  this  way  the  ground  became  very 
dry  and  hard  packed.  During  the  following  winter,  in  a  spasmodic 
effort  at  improvement,  a  large  stream  of  water  was  turned  into  the 
grove  and  the  whole  surface  completely  flooded  and  the  ground 
thoroughly  saturated.  Following  this  treatment  scaly  bark  appeared 
upon  several  hundred  of  the  trees  in  such  a  manner  that  one  could 
scarcely  doubt  the  connection  of  the  disease  with  this  radical  treatment. 
Further,  it  may  be  said  that  by  subsequent  good  cultivation  and  uniform 
irrigation  most  of  the  trees  recovered  and  came  back  into  good  condition. 

Relation  to  Gummosis. — Not  unfrequently  cases  occur  of  forms  of 
disease  intermediate  between  scaly  bark  and  gummosis.  In  these  the 
eruption  occurs  upon  the  trunk  with  a  scaling  off  of  the  bark  to  a  greater 
or  less  extent,  and  an  exudation  of  gum  in  a  manner  resembling  that 
in  both  of  the  typical  diseases  to  a  certain  extent.  In  fact,  all  gradations 
may  be  found  between  the  two  diseases.  Scaly  bark  has  all  the  appear- 
ance of  a  mild  form  of  gummosis,  affecting  the  tree  somewhat  differently 
on  acount  of  differences  in  conditions,  but  much  the  same  disease  in 
regard  to  its  real  nature.  The  fact  just  mentioned,  that  forms  inter- 
mediate between  the  two  diseases  can  be  found,  goes  far  to  prove  this, 
and  further  evidence  is  obtained  by  a  study  of  the  affected  tissues. 

The  condition  as  to  the  formation  of  the  gum  is  practically  the  same 
in  both  diseases,  the  only  difference  being  that  in  the  more  active 
gummosis  considerable  areas  of  bark  are  ruptured  from  the  wood,  while 
in  scaly  bark  the  gumming  is  less  active  and  breaks  out  only  rarely 
to  the  surface,  the  slight  irritation  set  up  causing  an  abnormal  growth 
of  wood  and  the  scaling  of  the  outer  bark.  The  effect  is  so  slight  that 
the  growth  of  the  wood  and  bark  at  the  affected  point  continues  for 
many  years,  the  wood  being  discolored  and  poorly  formed  on  account 
of  the  numerous  gum  pockets  which  it  contains,  and  the  bark,  rough, 
scaly,  warty,  and  impregnated  to  some  extent  with  the  dried  resinous 
gum.  The  presence  of  the  scaly  area  keeps  up  an  irritation  which 
extends,  rather  than  allowing  the  healing  of  the  wound. 

The  origin  of  the  gum  is  the  same  in  scaly  bark  as  in  gummosis,  but 
the  effect  on  the  tissues  is  somewhat  different.  In  scaly  bark  the 
abnormal  process  is  lass  active,  so  that  wood  formation  over  the  gum 
area  is  quickly  resumed.  This  results  in  the  continual  formation  of 
poorly  developed  wood  containing  numerous  gum  pockets  and  warty 
projections  which  push  out  the  bark  and  cause  the  typical  scale  forma- 
tion.    Scaly  bark,  therefore,  as  well  as  gummosis,  is  a  disease  of  the 


Bulletin  200. 


GUM    DISEASE   OF    CITRUS   TREES. 


261 


wood  rather  than  one  of  the  bark,  the  effect  on  the  latter  being  entirely 
secondary.  The  wood  of  scaly  bark  trees  is  discolored  beneath  the 
affected  areas,  extending  often  far  into  the  trunk  or  branch.  This 
discoloration  seems  to  spread  inward  to  some  extent   in  wood  which 


Fig.  12.  Schematic  sections  of  affected  stems  in  1,  Exanthema  ;  2,  Gummosis  ;  3,  Scaly 
Bark.  W  normal  wood,  C  cambium,  B  bark,  G  gum,  P  imperfect  wood.  In  3,  Scaly 
Bark,  normal  wood  is  seen  again  outside  the  gum  pocket,  forming  a  projection 
and  causing  a  pushing  out  and  scaling  of  the  bark.  The  gumming  region  is  com- 
posed of  a  mixture  of  gum  and  poorly  formed  wood.  In  2,  Gummosis,  a  solid 
mass  of  gum  is  seen  between  the  wood  and  bark.  In  1,  Exanthema,  the  pustule 
is  composed  largely  of  bark  tissue,  overlying  masses  of  abnormal  wood  in  which 
are  embedded  numerous  gum  pockets. 

had  previously  been  healthy,  but  is  usually,  if  not  always,  a  result  of 
the  growth  of  the  tree.  The  disease  is  not  severe  enough  to  kill  the 
tissue  entirely  at  the  affected  places,  so  that  the  formation  of  new  wood 
goes  on  to  some  extent,  and  this  wood  is  diseased  and  discolored  by 


262  UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION. 

gum  pockets,  and  after  a  number  of  years  may  have  considerable  thick- 
ness under  the  scaly  bark  spots. 

The  influences  which  determine  the  point  on  the  tree  where  scaly 
bark  may  break  out  have  been  already  alluded  to  on  page  240.  Of 
particular  influence  is  probably  the  irregular  growth  and  poor  devel- 
opment of  the  wood  under  uneven  moisture  conditions  affecting  the 
flow  of  sap. 

Relation  to  Florida  Diseases. — The  California  scaly  bark  disease  would 
seem  to  be  identical  with  the  trouble  mentioned  by  Swingle  and  Webber* 
as  occurring  in  Florida,  and  called  by  them  Psorosis.  They  speak  of 
this  disease  as  follows  "Psorosis,  the  disease  known  in  Florida  as  'tears' 
or  '  gum  disease, '  is  often  confounded  with  foot-rot,  but  is  unques- 
tionably quite  distinct.  In  appearance  it  is  similar  to  foot-rot,  but 
with  it  the  diseased  spots  occur  on  the  limbs  and  occasionally  on  the 
trunk,  but  never  on  the  roots,  so  far  as  known.  Psorosis  does  not  kill 
the  bark  entirely,  but  extends  only  to  the  middle  layer,  the  inner  bark 
and  cambium  layer  remaining  healthy."  In  the  annual  report  of  the 
Florida  Experiment  Station  for  the  year  ending  June  30,  1907,  two 
diseases  are  described,  either  one  of  which  might  be  identical  with  our 
scaly  bark.  The  one  described  on  page  43  under  this  same  name  and 
illustrated  in  plate  7  appears  much  the  same,  except  that  a  spotting 
of  the  fruit  is  described  as  a  symptom  of  the  disease,  which  is  not  the 
case  in  California.  On  page  46  of  the  same  report  a  disease  is  described 
under  the  name  of  "Gumming  of  Citrus,"  which  also  appears  to  have 
much  in  common  with  our  disease :  ' '  The  gumming  areas  are  usually  at 
a  considerable  distance  from  the  ground  on  the  trunk  and  larger 
limbs.  This  point  of  difference  distinguishes  this  from  the  foot-rot. 
The  trunk  cracks  and  gum  oozes  out  in  considerable  quantities.  New 
bark  tissue  is  formed  underneath  the  old.  The  surface  of  the  area 
becomes  characterized  by  ridges,  warts,  and  resinous  deposits.  Finally 
the  bark  dies  and  the  wood  slowly  decays  inward."  This  disease 
evidently  is  closely  related  to  one  or  the  other  of  the  forms  occurring 
in  California.  No  cause  is  ascribed  either  to  this  or  that  described  by 
Swingle  and  Webber.  The  disease  described  as  "scaly  bark"  in  the 
Florida  report  is  thought  to  be  infectious,  and  is  said  to  cause  a  serious 
dropping  of  the  fruit  as  well  as  the  spotting  already  mentioned. 

CONTROL  OF  SCALY  BARK. 

The  treatment  of  trees  badly  affected  with  scaly  bark  is  not  a  prom- 
ising undertaking.  When  far  advanced  the  condition  becomes  one  of 
such  a  chronic  nature  that  a  satisfactory  cure  is  almost  out  of  the 
question.     A  general  survey  of  the  occurrence  of  this  trouble  in  Cali- 

*  The  Principal  Diseases  of  Citrus  Fruits  in  Florida,  Bull.  8,  Div.  Veg.  Phys.  and 
Path.,  U.   S.  Dept.  of  Agr.   1896. 


Bulletin  200.  GUM  DISEASE  OF  CITRUS  TREES.  263 

fornia  makes  evident  the  fact  that  the  prevention  of  this  disease  is 
much  more  to  be  sought  after  than  methods  of  treatment  for  its  cure. 
Scaly  bark  is  not  a  disease  of  common  occurrence  in  our  best  orchards. 
While  occasionally  a  tree  may  be  affected  even  in  some  of  the  finest 
groves,  yet  such  cases  occur  very  largely  where  the  soil  is  of  uneven 
quality,  and  more  often  scaly  bark  accompanies  poor  culture,  unfavor- 
able soil  conditions,  or  neglect.  In  a  great  many  groves,  as  we  have 
already  mentioned,  where  the  present  methods  of  culture  are  good,  the 
scaly  bark  trees  are  a  relic  of  former  trouble  and  have  been  in  their 
present  condition  for  many  years. 

The  prevention  of  scaly  bark  appears  to  be  best  accomplished  by  the 
maintenance  of  uniformity  in  regard  to  moisture  conditions.  It  is 
scarcely  necessary  to  go  into  methods  of  accomplishing  this  in  this 
publication,  but  according  to  the  nature  of  the  soil  and  other  conditions 
the  grower  must  endeavor  to  maintain  uniform  moisture  rather  than 
to  allow  the  soil  to  become  alternately  wet  and  dry.  Frequent  exam- 
ination of  the  soil  by  digging  to  a  considerable  depth  in  different  parts 
of  the  orchard  will  be  found  a  most  profitable  practice  when  there  is 
any  uncertainty  as  to  the  moisture  condition. 

Treatment  of  Affected  Trees. — The  individual  treatment  of  affected 
trees  may  proceed  on  much  the  same  lines  as  that  described  for  gum- 
mosis.  It  is  useless,  however,  to  attempt  to  cut  out  or  strip  off  the 
affected  bark  if  the  disease  is  far  advanced.  When  the  scaly  areas  are 
stjll  small  they  may  be  cut  out,  taking  care  to  cut  all  the  discolored 
tissue  around  the  edges,  particularly  at  the  upper  end.  The  wound 
may  then  be  covered  with  the  wax  previously  described.  Where  large 
areas  of  scaly  bark  exist  on  the  tree  it  is  useless  to  try  to  cut  them  out 
and  in  most  cases  it  is  practically  impossible  to  do  anything  for  per- 
manent benefit.  If  the  condition  is  not  too  bad,  the  scaly  areas  may 
be  scraped  to  take  off  the  rough  bark,  leaving  the  green  inner  bark, 
most  of  which  is  alive.  A  few  slits  may  be  cut  through  this  to  allow 
the  escape  of  the  gum  and  the  place  then  painted  over  with  some 
softening  substance,  such  as  a  one-tenth  solution  of  potash  or  neat's- 
foot  oil,  as  previously  mentioned.  The  worst  affected  trees  may  profit- 
ably be  pulled  out  and  new  ones  planted  in  their  places,  after  first 
working  over  the  soil  deeply  to  make  sure  that  no  injurious  condition, 
such  as  hardpan  or  large  rocks,  exists  at  that  point.  Frequently  an 
affected  tree  can  be  cut  off  near  the  ground  below  the  scaly  bark 
areas,  and  upon  the  sprouts  which  come  up  buds  can  be  inserted  and 
a  new  tree  can  be  obtained  more  quickly  than  by  replanting.  If  these 
sprouts  come  from  above  the  original  point  of  budding  no  rebudding 
will  of  course  be  necessary. 


264  UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION. 


GUMMING  OF  NURSERY  TREES. 

It  is  quite  a  common  occurrence  for  young  citrus  trees  to  be  affected 
with  a  form  of  gum  disease  either  in  the  nursery  or  immediately  after 
planting  in  the  orchard.  This  trouble  consists  in  a  breaking  out  of 
gum  at  different  points  on  the  tree,  which  gum  may  be  easily  seen  to 
originate  from  beneath  the  bark,  breaking  out  through  ruptures  to  the 
surface.  If  the  gumming  occurs  soon  after  the  trees  have  been  pruned 
or  cut  in  any  way  it  usually  appears  at  the  point  where  the  cuts  have 
been  made.  Gumming  very  frequently  takes  place  at  the  point  where 
the  cut  in  the  bark  is  made  for  budding,  and  in  some  seasons  a  consid- 
erable loss  of  buds  is  experienced  in  this  way,  both  in  the  nursery  and 
in  older  trees  which  are  being  top-worked.  This  takes  place  particularly 
in  seasons  of  abundant  rainfall  in  the  spring.  Other  cases  occur  with 
young  trees  where  the  gum  appears  beneath  the  forks  and  crotches, 
at  the  junction  between  one  growth  and  another,  or  at  any  point  where 
the  flow  of  sap  is  somewhat  interfered  with.  This  trouble  almost  always 
occurs  under  the  conditions  just  mentioned  as  to  rainfall,  or  where  the 
trees  have  been  heavily  irrigated  and  the  water  allowed  to  stand  close 
about  them,  on  trees  planted  in  very  heavy  soil  and  often  where  the 
trees  are  planted  too  deeply  with  the  bud  union  covered  to  some  depth. 
The  whole  matter  of  gumming  of  young  trees  is  indeed  quite  an  obvious 
case  of  cause  and  effect,  illustrating  the  relation  between  citrus  gumming 
and  an  excess  of  water,  depth  of  planting  or  mechanical  injury.  A 
large  number  of  trees  show  more  or  less  gumming  during  the  first  year 
in  the  orchard,  but  usually  without  serious  consequences  if  they  are 
properly  attended  to  as  to  depth  of  planting  and  moisture  or  drainage 
conditions,  unless  they  have  been  too  badly  affected  to  recover.  Some 
trees  which  were  affected  in  the  nursery  may  continue  to  gum  in  the 
orchard,  even  under  good  conditions.  If  well  taken  care  of  they  will 
usually  recover. 

Control  of  Gum  Disease  in  Young  Trees. — In  the  nursery  gum  disease 
is  almost  always  caused  by  too  much  water  close  about  the  trees.  When 
due  to  rainfall  this  can  not  always  be  avoided,  but  the  choice  of  location 
for  the  nursery  should  be  one  of  good  drainage  and  not  too  heavy  soil. 
In  irrigation  the  water  should  not  be  allowed  to  flood  the  trees.  If  the 
trees  become  affected  in  the  nursery  they  should  be  kept  particularly 
well  cultivated  so  that  there  is  no  packing  of  the  soil  about  them,  the 
soil  should  be  kept  from  burying  the  trees  above  the  proper  depth,  and 
particular  pains  must  be  taken  to  prevent  water  from  standing  about 
them. 

When  young  trees  in  the  orchard  show  gumming,  the  same  treatment 
should  be  given  and  the  same  precautions  (xercised.     In  case  the  trees 


Bulletin  200.  GUm  DISEASE  OF  CITRUS  TREES.  265 

gum  when  everything  seems  all  right  in  these  respects  they  may  be 
still  showing  the  effects  of  disease  contracted  in  the  nursery,  and 
recover  of  themselves,  without  any  treatment.  If  a  considerable  amount 
of  gumming  occurs  on  the  trunk  of  the  young  tree  it  is  advisable  to 
relieve  this  condition  somewhat  by  slitting  the  bark  in  a  few  places,  as 
previously  described.  In  young  trees  it  is  not  advisable  to  strip  off  the 
whole  bark  at  the  gummed  places,  as  some  of  it  is  still  alive  and  might 
recover.  It  is  better  to  plant  a  new  tree  than  to  start  with  one  having 
a  large  amount  of  bark  removed. 

The  desirability  of  high-budded  nursery  trees  has  already  been  dis- 
cussed, and  the  general  course  of  treatment  given  on  page  247  applies 
here  as  well. 

FOOT-ROT. 

The  true  foot-rot,  as  described  in  Florida  and  many  foreign  countries, 
is  not  of  common  occurrence  in  California,  where  the  ordinary  forms 
of  gum  disease  are  evidently  of  a  somewhat  different  nature.  The  latter 
particularly  do  not  show  a  decay  of  the  roots,  a  rotting  of  the  bark  or 
any  effect  whatever  below  the  surface  of  the  ground.  The  Florida 
disease  is  decidedly  different  in  this  respect.  A  few  cases  of  true 
foot-rot  have  been  found  in  this  State,  and  these  agree  perfectly  with 
the  conditions  described  elsewhere.  In  some  instances  the  roots  were 
badly  decayed  from  the  surface  of  the  ground  downward,  even  before 
the  top  of  the  tree  showed  a  very  serious  condition.  The  decay  starts 
in  the  bark,  which  becomes  soft  and  rotten,  spreading  all  over  the  main 
roots  until  finally  the  tree  dies.  There  is  no  gumming  on  the  sides  of 
the  trunk  as  in  the  disease  which  we  have  described  as  gummosis.  The 
odor  of  the  affected  bark  is  disagreeable  and  quite  characteristic. 

In  the  cases  of  foot-rot  found  in  this  State  the  affected  trees  were 
invariably  growing  under  conditions  of  extremely  poor  drainage  and 
very  wet,  heavy  soil  about  the  roots.  As  the  same  condition  is  said 
to  exist  in  other  countries  where  this  disease  prevails,  it  would  appear 
to  bear  considerable  relation  to  the  other  forms  of  gum  disease  described. 
From  the  accounts  of  this  disease  in  many  countries  it  would  appear 
that  it  has  broken  out  in  an  epidemic  form,  spreading  rapidly  through 
the  citrus  districts  and  causing  widespread  destruction  of  groves  on 
all  kinds  of  soil.  For  this  reason  it  has  sometimes  been  thought  to  be 
caused  by  an  active  parasite  rather  than  to  be  simply  the  result  of 
unfavorable  conditions.  No  such  parasite  has  been  identified,  however, 
as  the  cause  of  the  disease  by  the  many  investigators  who  have 
studied  it,  and  the  conclusion  is  quite  general  that  its  relation  to  soil 
and  moisture  conditions  is  the  most  important  consideration  in  regard 
to  its  origin.     Certainly  in  California  the  cause  of  this  trouble  seems 


266 


UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION. 


to  be  very  plainly  an  extremely  heavy,  wet  condition  of  the  soil,  and 
the  disease  has  shown  no  evidence  of  spreading  by  infection. 

Control  of  Foot-rot. — The  control  of  foot-rot,  as  it  occurs  in  this 
State,  is  obviously  a  matter  of  improvement  in  drainage  conditions. 
Most  of  what  has  been  said  in  regard  to  gummosis  applies  equally  well 
here.  Affected  trees  may  be  treated  to  some  extent  by  the  methods 
described. 

FLORIDA  DIE-BACK,  OR  EXANTHEMA. 

The  disease  described  under  this  name  by  Swingle  and  Webber  iloc. 

cit.)  undoubtedly  exists  in  California  to  a  limited  extent.  Its  occur- 
rence, however,  is  so  mani- 
festly connected  with  certain 
soil  conditions  that  a  general 
spread  of  the  disease  is  not 
to  be  feared.  In  fact,  it  is 
evident  that  this  trouble  has 
occurred  in  California  for 
many  years,  since  it  is  de- 
scribed by  Mills  in  Bulletin 
138  of  the  California  Experi- 
ment Station,  issued  in  1902. 
The  disease  is  a  very  charac- 
teristic one,  and  is  in  many 
ways  quite  different  from  the 
other  forms  of  gum  disease 
described,  but  akin  to  them 
in  the  production  of  gum 
from  the  diseased  parts  and 
in  other  ways  which  we  shall 
show  later. 

The  most  prominent  char- 
acteristic of  exanthema  is  the 
occurrence  of  numerous 
swellings  or  pustules  on  the 
bark  of  the  smaller  branches 
and  twigs.  These  swellings 
are  very  numerous,  and  in 
bad  cases  the  twigs  and 
branches  are  completely  cov- 
ered with  them.    In  many  of 

the  eruptions  gum  pockets  are  formed  in  the  wood,  and  the  resinous 

gum  exudes  and  hardens  on  the  outside. 


Fig. 


13.   Exanthema  on  orange  twigs,  showing 
bark   pustules. 


Bulletin  200. 


GUM   DISEASE   OF    CITRUS   TREES. 


267 


Another  feature  of  the  disease  is  the  formation  of  a  large  number 
of  shoots  from  the  axils  of  the  twigs,  which  grow  out  in  a  bushy  manner 
but  soon  die.  The  ordinary  foliage  of  trees  affected  with  exanthema 
does  not  appear  sickly,  but  rather  the  opposite.  The  leaves  become 
abnormally  large  and  of  a  very  dark  green  color,  giving  a  deceptive 
appearance  of  health.  In  the  fruit  of  affected  orange  trees  extremely 
characteristic  symptoms  of  exanthema  are  found.  The  normally  green 
color  of  the  unripe  orange  becomes  pale  and  unhealthy  looking,  and 
develops  as  the  fruit  matures  into  a  peculiar  lemon  yellow  or  doll, 
yellowish  green,  quite  different 
from  any  color  ever  seen  in 
healthy  fruit.  The  oranges  thus 
affected  become  stained  with  a 
brown,  gummy  exudation  oc- 
curring in  patches  on  the  rind. 
At  these  places  the  fruit  usually 
splits  and .  finally  falls  from  the 
tree  before  reaching  maturity. 
This  effect  on  the  fruit  appears 
to  be  one  of  the  first  indications 
of  the  disease  in  California,  oc- 
curring sometimes  without  any 
of  the  symptoms  mentioned  on 
the  twigs,  and  taking  place  one 
year  with  no  indication  of  the  dis- 
ease whatever  during  the  follow- 
ing season.  This  happens  only 
in  the  regions  to  which  the  typ- 
ical occurrence  of  the  disease  is 
confined.  The  affected  fruit  has 
very  little  acidity,  even  when 
small  and  immature,  but  an  in- 
sipid sweetness  long  before  its 
natural  time  of  ripening.  The 
staining  of  the  fruit  is  caused  by 
a  deposit  of  a  gummy  substance 
in  the  cells  of  the  epidermis. 

Relation  to  Soil. — The  occurrence  of  exanthema  in  southern  California 
is  limited  almost  absolutely  to  groves  on  the  very  porous,  granitic,  rocky 
type  of  soil  which  occurs  close  to  the  base  of  the  mountains  in  sOme 
districts.  Under  such  conditions  the  disease  is  prevalent  and  abundant, 
and  nowhere  else.  Its  only  occurrence  beyond  this  has  been  in  a  very 
few  isolated  instances. 


Fig.  14.     Exanthema,    showing  dead, 
bushy   growth. 


268  UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION. 

Relation  to  Fertilization. — The  cause  of  exanthema  is  thought  in 
Florida  to  be  the  excessive  use  of  organic  nitrogenous  fertilizers.  The 
injudicious  application  of  materials  of  this  sort  is  said  to  be  commonly 
followed  by  the  appearance  of  the  disease.  To  the  same  cause  may  be 
ascribed  the  occurrence  of  exanthema  in  California  in  the  few  isolated 
cases  mentioned,  not  on  the  type  of  soil  described  as  showing  the  most 
of  the  trouble.  In  these  individual  cases  excessively  heavy  applications 
of  sheep  manure  or  other  organic  material  had  been  made  and  the 
typical  disease  resulted.  It  may  also  be  said  that  after  this  practice 
was  stopped  the  trouble  disappeared.  *  On  the  soil  where  exanthema  is 
most  common  no  excessive  amount  of  fertilizer  of  any  sort  can  be  con- 
sidered the  cause  of  the  trouble.  It  is  true,  however,  that  to  a  consid- 
erable extent  the  disease  appears  upon  trees  which  have  received  a  fair 
amount  of  plant  food,  while  trees  upon  the  same  soil  which  have  not 
been  well  nourished  show  ordinary  die-back  and  starvation,  but  not  the 
characteristic  exanthema.  Experience  is  also  reported  upon  this  soil 
that  the  spasmodic  application  of  considerable  amounts  of  stable  manure 
has  produced  the  symptoms  of  the  disease  in  the  fruit  even  when  the 
trees  were  not  badly  affected. 

Cause. — It  seems  most  probable  that  the  occurrence  of  exanthema 
almost  exclusively  upon  this  type  of  soil  is  connected  with  the  irregu- 
larity in  the  food  and  moisture  conditions  to  which  the  trees  are  sub- 
jected. During  irrigation  the  extreme  looseness  of  the  soil  makes  the 
water  and  its  dissolved  mineral  salts  freely  available  to  the  roots,  and 
the  movement  of  soil  water  and  salts  is  very  free.  Unless  the  moisture 
condition  of  the  soil  is  very  closely  watched,  however,  it  is  likely  to 
dry  out  rapidly  and  there  is  also  very  little  lateral  penetration  from 
the  irrigation  furrows.  The  whole  condition  is  one  likely  to  produce 
extremely  irregular  nutrition  of  the  tree,  unless  very  carefully  handled. 
It  would  seem  that  in  this  factor  lies  the  reason  for  the  occurrence  of 
exanthema  upon  this  soil.  Manifestly  a  very  careful  attention  to  the 
maintenance  of  uniform  moisture  and  fertility  is  most  important  in 
the  control  of  the  disease,  and  this  in  actual  practice  has  proven  to  be 
the  case. 

Very  large  amounts  of  organic  fertilizing  materials,  such  as  blood 
or  stable  manure,  must  be  used  cautiously  in  view  of  this  disease, 
yet  at  the  same  time  the  type  of  soil  on  which  the  trouble  is  most 
common  is  particularly  in  need  of  just  such  material.  The  whole 
trend  of  the  logical  handling  of  this  soil  is  along  the  line  of  increased 
organic  matter  in  order  to  increase  the  water-retaining  capacity  and  thus 
promote  uniform  growth.  Without  the  addition  of  considerable  amounts 
of  such  material  in  the  form  of  manure  and  green  manure  crops,  it  is 
most  difficult  to  maintain  uniform  moisture  conditions,  while  at  the 


Bulletin  200.  GUM  DISEASE  OF  CITRUS  TREES.  269 

same  time  supplementary  applications  of  commercial  fertilizer  are  neces- 
sary for  the  sake  of  plant  food. 

For  avoiding  exanthema  on  these  coarse,  rocky  soils,  as  well  as  for 
successful  citrus  production  in  general  in  the  same  locality,  the  object 
must  be  to  increase  the  organic  matter  and  thereby  the  water-holding 
capacity  of  the  soil  by  the  use  of  stable  manure  and  green  manure 
crops,  to  feed  the  trees  liberally,  particularly  with  nitrogen  and  phos- 
phoric acid,  and  at  the  same  time  keep  the  soil  uniformly  moist  by 
careful  attention  to  the  methods  of  irrigation  and  cultivation  practiced. 

The  condition  which  tends  most  to  produce  the  disease  is  that  of 
extreme  dryness  between  irrigations,  which  often  occurs  in  this  soil. 
This  is  due  to  the  extreme  porosity,  causing  the  water  to  quickly  sink 
downward  and  leach  away,  accompanied  by  rapid  evaporation  from  the 
surface.  To  wet  this  soil  uniformly  and  keep  it  moist  the  runs  of  water 
must  be  short,  the  irrigations  frequent  and  abundant,  and  the  furrows 
numerous  and  covering  the  whole  surface  of  the  ground.  Long  runs  of 
water,  few  furrows,  and  wide  spaces  between  them  and  scanty  applica- 
tions are  to  be  particularly  avoided.  Frequent  examination  by  digging 
will  readily  show  the  moisture  condition  of  the  soil. 

The  prominent  formation  in  the  wood  of  rings  of  growth,  mentioned 
in  connection  with  scaly  bark,  is  even  more  marked  and  very  striking 
in  exanthema,  pointing  to  the  same  condition  of  irregular  growth. 

Effect  of  Spraying. — Considerable  discussion  has  been  provoked  con- 
cerning the  alleged  cure  or  improvement  of  trees  affected  with  exan- 
thema by  spraying  the  foliage  with  Bordeaux  mixture.  The  results 
particularly  of  such  spraying  carried  on  in  the  vicinity  of  West  High- 
lands by  Mr.  D.  W.  Divine  point  strongly  to  decided  improvement  in 
the  condition  of  the  trees.  From  the  well  known  physiological  action 
of  Bordeaux  mixture  upon  foliage,  aside  from  its  effect  as  a  fungi- 
cide, and  the  apparent  nature  of  the  disease  as  described  above,  it  is 
not  irrational  to  attribute  the  beneficial  results  claimed  to  the  action 
of  the  spray.  It  has  frequently  been  shown  that  the  action  of  the 
mixture  on  foliage  is  to  increase  and  stimulate  assimilation,  producing 
decidedly  beneficial  results  in  this  way,  even  on  healthy  plants  not 
attacked  by  any  parasite  or  disease.  This  being  the  case  it  is  not 
improbable  that  Bordeaux  mixture  applied  to  the  leaves  of  orange  trees 
affected  by  exanthema  acts  as  a  stimulant  to  the  nutritive  processes 
and  thus  counteracts  the  injurious  tendencies  of  the  conditions  described. 
For  permanent  benefit,  however,  it  is  necessary  to  pay  the  strictest 
attention  to  obtaining  the  best  possible  cultural  conditions  before  resort- 
ing to  spraying  or  any  similar  expedient. 


270  UNIVERSITY   OF   CALIFORNIA EXPERIMENT   STATION. 


TWIG   BLIGHT. 

A  condition  occasionally  occurs  on  orange  trees  in  California  which 
may  be  designated  by  this  name.  This  is  seen  as  a  sudden  dying  and 
withering  of  small  terminal  twigs  here  and  there  in  the  tree  which  had 
hitherto  appeared  entirely  healthy  and  normal.  The  green  leaves  sud- 
denly wither  and  remain  attached  to  the  twig,  which  dies  back  for  a 
short  distance,  and  with  its  dead  leaves  shows  very  prominently  in  the 
green  foliage  of  the  tree.  Usually  only  small  twigs  are  affected  and 
the  appearance  is  as  though  they  had  been  broken  and  remained  hang- 
ing. At  the  point  to  which  the  twig  dies  back  a  few  drops  of  gum  appear 
and  harden.  This  trouble  is  by  no  means  a  serious  or  important  one 
and  need  not  be  apprehended  in  the  least  where  it  occurs.  It  is  quite 
a  common  occurrence,  but  never  results  seriously.  No  apparent  cause 
can  be  seen  for  the  death  of  the  twigs  in  this  manner,  but  the  production 
of  gum  would  indicate  an  interference  with  the  flow  of  sap  for  some 
reason. 

CITRUS   GUMMING  IN   GENERAL. 

As  may  be  seen  from  the  description  of  the  diseases  given  above,  the 
production  of  gum  is  the  almost  constant  accompaniment  of  any  injury 
to  or  interference  with  the  functions  of  the  citrus  tree.  Beside  the  cases 
mentioned  others  might  be  described  where  gumming  occurs,  but  those 
given  are  the  most  characteristic. 

Orange  Splitting.— In  the  splitting  of  oranges,  particularly  the  Navel, 
which  is  so  abundant  and  destructive  some  seasons,  a  drop  of  hardened 
gum  is  invariably  found  just  at  the  spot  where  the  split  in  the  tissue 
occurs.  Whether  this  accumulation  of  gum  is  a  cause  or  effect  of  the 
splitting  is  difficult  to  determine.  Indeed,  in  many  oranges  which  do 
not  split,  but  mature  normally,  drops  of  hardened  gum  can  be  found 
at  the  blossom  end.  The  same  is  seen  regularly  in  the  saucer  peach, 
grown  commonly  in  southern  California. 

Leaf  Gumming. — The  leaves  of  oranges  and  other  citrus  trees  often 
show  a  form  of  gumming,  but  not  as  an  indication  of  any  particular 
disease.  In  many  cases,  indeed,  trees  with  such  leaves  are  to  all  appear- 
ances entirely  healthy.  This  gumming  takes  the  form  of  a  brown  solid 
deposit  in  spots  or  pustules  just  beneath  the  surface.  Trees  which  are 
affected  with  any  injurious  condition  are  likely  to  show  this  effect  in 
the  leaves,  but,  as  just  said,  it  is  not  always  an  indication  of  any  trouble. 


STATION   PUBLICATIONS.  271 


STATION  PUBLICATIONS  AVAILABLE  FOR  DISTRIBUTION. 


REPORTS. 


189G.     Report    of    the    Viticultural    Work    during    the    seasons    1887-93,    with    data 

regarding  the  Vintages  of  1894-95. 
1S97.     Resistant    Vines,    their    Selection,    Adaptation,    and    Grafting.      Appendix    to 

Viticultural  Report  for  1896. 
1S9S.     Partial  Report  of  Work  of  Agricultural   Experiment  Station   for   the  years 

1895-96  and  1896-97. 
1900.     Report  of  the  Agricultural  Experiment  Station  for  the  year  1897-98. 

1902.  Report  of  the  Agricultural  Experiment  Station  for  1898-1901. 

1903.  Report  of  the  Agricultural  Experiment  Station  for  1901-1903. 

1904.  Twenty-second  Report  of  the  Agricultural  Experiment  Station  for  1903-1904. 


TECHNICAL   BULLETINS— ENTOMOLOGICAL  SERIES. 


Vol.  1,  No.  1 — Wing  Veins  of  Insects. 

No.  2 — Catalogue  of  the  Ephydridae. 


BULLETINS. 

Reprint.  Endurance  of  Drought  in  Soils  of  the  Arid  Region. 

No.  12S.  Nature,  Value  and  Utilization  of  Alkali  Lands,  and  Tolerance  of  Alkali. 
(Revised  and  Reprint.  1905.) 

133.  Tolerance  of  Alkali  by  Various  Cultures. 

140.  Lands  of  the  Colorado  Delta  in  Salton  Basin,  and  Supplement. 

141.  Deciduous  Fruits  at  Paso  Robles. 

142.  Grasshoppers  in  California. 

147.  Culture  Work  of  the  Substations. 

148.  Resistant  Vines  and  their  Hybrids. 

149.  California  Sugar  Industry. 

150.  The  Value  of  Oak  Leaves  for  Forage. 

151.  Arsenical  Insecticides. 

152.  Fumigation  Dosage. 

153.  Spraying  with  Distillates. 

154.  Sulfur  Sprays  for  Red  Spider. 
156.  Fowl  Cholera. 

158.  California  Olive  Oil ;  its  Manufacture. 

159.  Contribution  to  the  Study  of  Fermentation. 

160.  The  Hop  Aphis. 

161.  Tuberculosis  in  Fowls.      (Reprint.) 

162.  Commercial  Fertilizers.      (Dec.  1,  1904.) 

163.  Pear  Scab. 

164.  Poultry  Feeding  and  Proprietary  Foods.     (Reprint.) 

165.  Asparagus  and  Asparagus  Rust  in  California. 

166.  Spraying  for  Scale  Insects. 

167.  Manufacture  of  Dry  Wines  in  Hot  Countries. 

168.  Observations  on  Some  Vine  Diseases  in  Sonoma  County. 

169.  Tolerance  of  the  Sugar  Beet  for  Alkali. 

170.  Studies  in  Grasshopper  Control. 

171.  Commercial  Fertilizers.     (June  30,  1905.) 

172.  Further  Experience  in  Asparagus  Rust  Control. 

173.  Commercial  Fertilizers.     (December,  1905.) 

174.  A  New  Wine-Cooling  Machine. 

175.  Tomato  Diseases  in  California. 

176.  Sugar  Beets  in  the  San  Joaquin  Valley. 

177.  A  New  Method  of  Making  Dry  Red  Wine. 

178.  Mosquito  Control. 

179.  Commercial  Fertilizers.     (June,  1906.) 

180.  Resistant  Vineyards. 

181.  The  Selection  of  Seed-Wheat. 

182.  Analysis  of  Paris  Green  and  Lead  Arsenate.     Proposed  Insecticide  Law. 

183.  The  California  Tussock-moth. 


272 


STATION   PUBLICATIONS. 


No.  184.  Report  of  the  Plant  Pathologist  to  July  1,  1906. 

185.  Report  of  Progress  in  Cereal  Investigations. 

186.  The  Oidium  of  the  Vine. 

187.  Commercial  Fertilizers.      (January,  1907.) 

188.  Lining  of  Ditches  and  Reservoirs  to  Prevent  Seepage  and  Losses. 

189.  Commercial  Fertilizers.     (June,  1907.) 

190.  The  Brown  Rot  of  the  Lemon. 

191.  California  Peach  Blight. 

192.  Insects  Injurious  to  the  Vine  in  California. 

193.  The  Best  Wine  Grapes  for  California ;   Pruning  Young  Vines ;    Pruning 

the   Sultanina. 

194.  Commercial   Fertilizers    (Dec.   1907). 

195.  The  California  Grape  Root-worm. 

196.  Eucalyptus  in  California. 

197.  Grape  Culture  in  California  ;  Improved  Methods  of  Wine  Making  ;  Yeasts 

from  California  Grapes. 

198.  The  Grape  Leaf-Hopper. 

199.  The  Bovine  Tuberculosis. 


CIRCULARS. 


No.  1. 
2. 
3. 
4. 
5. 
7. 
9. 
10. 

11. 
12. 
13. 
15. 

16. 
17. 

18. 
19. 
20. 
21. 

22. 

23. 
24. 
25. 


Texas  Fever. 

Blackleg. 

Hog  Cholera. 

Anthrax. 

Contagious  Abortion  in  Cows. 

Remedies  for  Insects. 

Asparagus  Rust. 

Reading  Course  in  Economic 
Entomology.      (Revision.) 

Fumigation  Practice. 

Silk  Culture. 

The  Culture  of  the   Sugar  Beet. 

Recent   Problems   in   Agriculture. 
What  a  University  Farm  is  For. 

Notes  on  Seed-Wheat. 

Why      Agriculture      Should      Be 
Taught  in   the  Public  Schools. 

Caterpillars  on  Oaks. 

Disinfection   of  Stables. 

Reading  Course  in  Irrigation. 

The  Advancement  of  Agricultural 
Education. 

Defecation    of    Must    for    White 
Wine. 

Pure  Yeast  in  Wineries. 

Olive  Pickling. 

Suggestions   Regarding   Examina- 
tion  of   Lands. 


No.  26. 


28. 


29. 


30. 
31. 


32. 
33. 

34. 

35. 


36. 


Selection  and  Preparation  of 
Vine  Cuttings. 

Marly  Subsoils  and  the  Chlo- 
rosis or  Yellowing  of  Citrus 
Trees. 

A  Preliminary  Progress  Report 
of  Cereal  Investigations,  1905- 
1907. 

Preliminary  Announcement  con- 
cerning Instruction  in  Practi- 
cal Agriculture  upon  the 
University  Farm,  Davisville, 
Cal. 

White   Fly   in  California. 

The  Agricultural  College  and  Its 
Relationship  to  the  Scheme  of 
National    Education. 

White  Fly  Eradication. 

Packing  Prunes  in  Cans.  Cane 
Sugar  vs.   Beet   Sugar. 

California  State  Farmers'  Insti- 
tute at  the  University  Farm 

Southern  California  Patholog- 
ical Laboratory  and  Citrus 
Experiment  Station. 

Analyses  of  Fertilizers  for  Con- 
sumers. 


Copiet  may  be  had  on  application  to  Directob  of  Expebiment  Station,  Berkeley,  Cal. 


